At a glance
| Criterion | IP PBX | Centrex / cloud phone system | AI voice agent (Alyas) |
|---|---|---|---|
| Where the system runs | The company’s own server, physical or virtual | The operator’s or software vendor’s platform | Service hosted in the European Union |
| Main role | Switching calls between extensions | Switching calls with no central equipment on the premises | Answering and holding a conversation with the caller |
| Who answers the caller | Your staff | Your staff | The AI, with transfer when needed |
| Team handsets and softphones | Yes | Yes | No, outside its scope |
| Operation and updates | In-house team or integrator | Service provider | Alyas |
| Typical pricing model | Purchase or lease, plus maintenance | Subscription per user or per line | Monthly plan with included minutes |
| Link with an AI agent | Call forwarding to the agent | Call forwarding to the agent, or a built-in AI agent offered as an option by some providers | Receives forwarded calls |
The IP PBX: the phone system you own
An IP PBX (IPBX) is a private branch exchange that runs over IP. It is switching software installed on a company server, or on a virtual machine the company administers, and connected to the public network by an operator, for example via a SIP trunk.
Its appeal lies in control: the numbering plan, routing rules and data remain in the company’s hands. The trade-off is running it: updates, security, backups and outages are down to the in-house team or an integrator.
Centrex: the phone system you rent
With Centrex, the switching functions run on an operator’s platform. The company only installs IP handsets or softphones and pays a subscription, usually per user or per line.
What is now called a cloud or virtual phone system follows the same logic, whether it is sold by a telecom operator or by an online telephony vendor. Administration is done through a web interface, with no server to maintain on the premises.
VoIP: a technology, not a type of phone system
Voice over IP means carrying the conversation as data packets. IP PBX and Centrex both rely on it, so the term “business VoIP” can cover, depending on the provider, a SIP line, a hosted phone system or a complete IP telephony package.
Choosing VoIP therefore does not settle the architecture question: you still have to decide whether the phone system is hosted on your own premises or by a provider.
Where an AI voice agent fits in
An AI voice agent does not switch calls between internal extensions: it answers them. Alyas picks up, announces that it is an AI and that the call is recorded, answers using the information provided by the business, takes contact details, transfers to a configured number if needed and produces a transcript and a summary of every call.
It connects through call forwarding, a common feature on IP PBXs and Centrex offers alike; the available scenarios (immediate forwarding, on no answer, when busy, outside business hours) vary with the equipment or package. The existing phone system stays in place for the team; the agent takes the calls the team cannot. Our AI phone system page explains how this works in detail.
Some providers now build an AI voice agent into their cloud phone system, such as AIRO from Ringover or Mon Standard Intelligent from the French operator Bouygues Telecom Pro. For a business that is already a customer of one of them, this built-in option is worth comparing with an independent agent.
When the agent can be enough without a dedicated phone system
A business with just one line and a mobile does not necessarily need to invest in an IP PBX or Centrex. Forwarding that line to the agent, which transfers to the mobile the calls that need to be handled live, can cover most of its call-answering needs.
As soon as several people need to answer, transfer calls to one another or work with call queues, a real phone system is still needed. Alyas does not provide one: it works alongside it.
Our verdict
A business with a technical team and demanding control requirements will turn to an IP PBX; one that wants a phone system with no server to manage will choose Centrex or a cloud phone system. A voice agent like Alyas is added to either one for unanswered calls, and can be enough on its own for a very small business.
Frequently asked questions
What is the difference between an IP PBX and Centrex?
An IP PBX is a phone system installed and run by the business on its own server. Centrex is a phone system hosted and maintained by an operator and rented on subscription. Both use voice over IP.
Do I need to change my phone system to use an AI voice agent?
No. The agent connects through call forwarding, a common feature on IP PBXs and Centrex offers. Depending on your phone system’s forwarding options, you choose which calls are forwarded to it: all of them, unanswered calls or out-of-hours calls.
Can an AI voice agent replace a cloud phone system?
For a team, no: Alyas provides no softphone, no handsets and no multi-agent call queues. For a sole trader or a very small business whose calls come in on a single line, it can cover call answering without a dedicated phone system.