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Telephony · Understand

VoIP, SIP, IP PBX and UCaaS: which architecture

VoIP is a way of transporting voice, SIP a protocol, an IP PBX a phone system, UCaaS a service. These terms cannot be compared with one another. The real decision involves three points: who operates the phone system, whether it is shared with other customers, and how it stays reachable when a site or a link goes down.

Updated October 20267 minOfficial sources cited

Four terms, four levels

VoIP (Voice over IP): the voice is split into packets and carried over an IP network, the Internet or a private network, instead of a traditional phone line. It is a means of transport, not a product.

SIP (Session Initiation Protocol): the protocol standardized by the IETF in RFC 3261 for establishing, modifying and terminating sessions, including calls. It is a signaling protocol: it organizes the call. The voice itself generally travels over another protocol, RTP, or its secure version SRTP.

IP PBX: the company’s phone system, in its IP version. It manages extensions, direct numbers, groups, the auto attendant, call queues and forwarding. It can run on your premises or at a third party.

UCaaS (Unified Communications as a Service): a suite hosted and operated by a third party, often combining voice, presence, instant messaging and video conferencing.

Two more terms complete the buying vocabulary. A SIP trunk is the connection of your phone system to the public telephone network, provided by a carrier. A cloud PBX, or hosted IP Centrex, is a hosted phone system shared among several customers, each seeing only its own part.

Remember one rule: an IP PBX can be on your premises or at a third party; UCaaS is always a service operated by a third party. Many UCaaS offerings are in fact built on an IP PBX, plus a set of applications around it.

Shared cloud PBX or dedicated IP PBX

Both are hosted. They do not answer three questions in the same way.

QuestionShared cloud PBXHosted dedicated IP PBXOn-premises IP PBX
Who administers itThe provider; you configure your partThe provider or you, depending on the contractYou or your installer
Neighbors on the machineOther customersNoneNone
Power outage at the officeThe phone system remains reachableThe phone system remains reachableThe phone system stops, unless there is a UPS
UpdatesProvider, at the platform’s paceProvider, can be scheduled for youYou or your installer
ExitExport as provided for by the platformIsolated configuration and data, easier to extractEverything is on your premises

A dedicated system isolates the organization: a configuration error or an overload at another customer does not affect it, and data is not mixed. It does not change the question of who the operator is. A dedicated server is still operated by someone, who can restore it and can therefore access it.

Should you host your IP PBX yourself

Hosting the phone system on your premises gives you control over the hardware, the network and the timing of updates. That comes at a price, which must be read in full.

  • A local outage becomes a phone system outage. Power cut, fire, Internet link down: incoming calls no longer reach anyone, unless forwarding has been set up with the carrier.
  • Operations are your responsibility. Security patches, backups, monitoring for abnormal usage, hardware replacement. An exposed and forgotten IP PBX is a fraud target, see Securing IP telephony and limiting fraud.
  • The connection remains external. Outgoing calls still go through a carrier.

Self-hosting is not automatically more sovereign. If a provider administers the server remotely, it has access just as a hosting provider would. The guide What is a sovereign cloud? sums it up: what matters is the operator, the law that applies to it and the exit, more than the building.

On-premises installation is justified when a network must remain isolated, when data must not leave a location, or when an in-house team knows how to operate it.

It is poorly suited to a small organization with no one to keep up with updates, or to teams that are often out of the office.

Telephony without depending on a foreign cloud

The question is not about the term VoIP. It is about two things: the company that operates the phone system, and the path the communications take.

For the operator, the criteria are those of any data entrusted to a third party: its nationality, the law that applies to it, its subcontractors, the traceability of its access. They are developed in Protecting a company from the CLOUD Act and European cloud or American cloud.

For the path, one limitation must be accepted: a call to an external party crosses carrier networks that neither you nor your provider control. You can choose the phone system’s operator and encrypt the path up to it. The rest of the route depends on the carriers.

Multiple sites: one phone system or several

Two setups are possible.

  • One central phone system for all sites. A single numbering plan, a common reception, transfers between sites as between two neighboring offices, a single point of administration. If the phone system is hosted, each site connects to it over the Internet.
  • One phone system per site, or per group of sites, linked together. Each site keeps its autonomy and internal calls pass from one server to another without a carrier. Administration and a single reception, however, require more preparation.

Three criteria help you choose.

  1. Survivability: what happens if a site’s link goes down? With a hosted phone system, the isolated site can no longer call over the Internet, but its numbers remain reachable and can be forwarded to mobiles. See Keeping telephony running during an outage.
  2. Numbering plan: direct numbers per site, short internal extensions, a single reception number or one per branch.
  3. Reception: a single phone system that routes to the right site, or a local reception per site.

How to actually connect multiple sites to Klytic Tel is described in Connecting multiple sites to Klytic Tel.

Frequently asked questions

Are SIP and VoIP the same thing?

No. VoIP refers to transporting voice over IP. SIP is the protocol that sets up and ends calls in most IP phone systems.

Is UCaaS more secure than an IP PBX?

Neither more nor less by nature. It all depends on the operator, the isolation of your data and the anti-fraud controls. See Securing IP telephony.

Do you need to change carriers to change phone systems?

Not necessarily. An IP phone system can connect to the existing carrier if the connection is compatible.

Can we keep our numbers?

Yes, through number porting, which transfers a number from one carrier to another. See Connecting multiple sites to Klytic Tel.

Where Klytic fits

Klytic Tel is a dedicated IP PBX, operated by Klytic, with applications around it. Each organization has its own phone server. It is hosted by default in a European cloud, in Mauritius or in a geographic zone compliant with the applicable jurisdiction, at your request, or installed on your servers. In all cases, Klytic operates it. In the cloud, Klytic Tel relies on a leading cloud hosting provider, with an enterprise-grade availability commitment of 99.99% (excluding planned maintenance announced in advance) and failover between several hosting sites in the event of an outage: resilience to outages is very high. Klytic can also deploy the solution with another cloud provider of your choice. Klytic is operated by Dedicace Software, a French company.

Four line types can be combined on this server, each including the previous one:

  • Basic Line for a reception, workshop or shared phone: call and be called, voicemail, call queues, recording.
  • Advanced Line for teamwork: several devices per person, apps, presence, chat, internal video conferencing.
  • Business Line for those who talk to customers: external chat and video, transcription and summaries, SMS, connection to business tools including Klytic CRM.
  • Manager Line for oversight: statistics, dashboards, supervision, contact center.

A physical phone is not required; desk phones are available for rent and replaced if broken. Your numbers can be ported. You can keep your carrier, subject to compatibility, or take Klytic’s landline and mobile calling plans. For multiple sites, Klytic Tel works with a single server for all sites, or with several servers, each associated with certain sites and connected to one another, which makes internal calls between sites free and secure.

Klytic Tel is not a SecNumCloud-qualified offering. Nor is it a feature-for-feature equivalent of an American communications suite: compare based on your actual usage.

Sources

Consulted in October 2026.

  • IETF, RFC 3261, “SIP: Session Initiation Protocol” (June 2002): SIP is an application-layer signaling protocol, used with other protocols such as RTP and SDP to form a complete multimedia architecture. rfc-editor.org
  • IETF, RFC 3711, “The Secure Real-time Transport Protocol (SRTP)” (March 2004): an extension of RTP that provides confidentiality, message authentication and replay protection. rfc-editor.org
  • Klytic, Klytic Tel page: dedicated server per customer, four line types, keep your carrier or use Klytic plans, number porting, multi-site. klytic.com
  • Klytic, Hosting & sovereignty: deployment modes and zones, Klytic Tel hosted on a dedicated server. klytic.com
  • Klytic, “What is a sovereign cloud?”: the operator and the applicable law matter more than the location; limits of telephony encryption toward external networks. klytic.com

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Further reading

Use · TelephonyConnecting multiple sites to Klytic TelOne dedicated server and one phone system for all your branches: what you prepare with Klytic, how each site connects, and porting your numbers.5 min readRead →Understand · TelephonySecuring IP telephony and limiting fraudPhone fraud is paid for in calls to premium-rate numbers, often outside business hours. What is exposed, the controls, the signs of a hijacked account.7 min readRead →Understand · TelephonyKeeping telephony running during an outageBuilding, Internet link or platform: each telephony outage calls for a different fallback. What a backup restores, and what it does not replace.7 min readRead →

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