A VoIP phone system for a small business: numbers, porting and AI receptionists

A VoIP phone system carries your business calls over the internet instead of a traditional phone line, so your numbers live in an online account and ring on desk phones, computers and mobiles wherever your team is. For a small business, the choice comes down to three practical things: keeping the numbers customers already know, controlling where calls go when nobody answers, and whether the system can pass calls to other tools, such as an AI receptionist.

This article covers the common setups, how number porting works, emergency calling, and exactly what an AI receptionist needs from your phone system. CentriWeb builds and looks after voice agents for clients, on platforms that connect to phone systems like these, so weigh our view with that in mind. Vendor details below are from their own pages, as at November 2026.

Three common VoIP phone system setups

VoIP stands for voice over internet protocol. Most small businesses end up with one of these three, and some combine them.

Microsoft’s Teams Phone connectivity page shows the range within one product. A Calling Plan has Microsoft as your phone company. Operator Connect uses a certified third-party operator. Direct Routing connects any phone company through a session border controller that you, an integrator or a service provider runs. Microsoft notes that a Calling Plan needs a Calling Plan licence. Check their page for current prices.

Your numbers: what to sort out first

Customers know your number from your website, signage, vans and Google listing. Before you choose a system, list every number you use:

• the main business line;
• direct lines and mobile numbers staff give to customers;
• toll-free numbers;
• tracking numbers used in advertising; and
• any number a fax, alarm or card terminal still uses.

For each, note which provider holds it and whose name the account is in. A number held in a former employee’s or an old supplier’s account is hard to move. Fix the account holder first.

Porting: keeping your existing numbers

Porting moves a number from your current provider to a new one. The rules differ by country, and each provider has its own process. Two vendor pages show what the paperwork looks like.

Microsoft’s page on transferring numbers to Teams lists country-specific documents and Letter of Authorisation forms for countries including Australia, New Zealand, Canada, the United Kingdom and the United States. It says:

• your organisation name, account number and service address must match the current provider’s records exactly;
• porting every number on the account, including the billing number, closes the account with the current provider;
• the authorised user on your current account signs the Letter of Authorisation;
• an account PIN is needed to port mobile numbers; and
• in its United States and Canada porting wizard, Microsoft acknowledges a request within 72 business hours.

For United States numbers, Twilio’s port-in documentation says it generates an electronic Letter of Authorisation for the authorised representative to sign, asks for utility bills as proof of ownership, and cancels the request if it isn’t signed within 30 days.

Practical advice, wherever you are: keep the old service running until the new provider confirms the port is complete, schedule the switch for a quiet day, and test calling every number from a mobile afterwards.

Emergency calls and your address

An internet phone can be used anywhere, so the system needs to know where each phone is. Microsoft’s guidance on emergency calling says assigning an emergency address to each user ensures precise location information goes with an emergency call.

Some providers charge for this. Twilio’s United States SIP trunking pricing lists emergency calling at $0.75 per number per month, and a $75 fee per 911 call from a number with no registered emergency address. Emergency numbers and rules differ by country, so ask your provider how emergency calls work on your plan and where you register your address.

What an AI receptionist needs from your phone system

An AI receptionist answers calls, takes details, books appointments and hands over to a person. Our guide to AI receptionists for small business covers what they do, and how an AI voice agent works explains what happens on the call. From the phone system, it needs four things.

1. A number to answer on

Either the AI platform provides a number and your main number forwards to it, or your existing number is pointed at the AI platform directly. Forwarding is quicker to set up and easy to undo. Porting or pointing the number directly gives more control.

2. Forwarding rules you control

Most businesses don’t send every call to the AI. Common rules are: forward when nobody answers after a set number of rings, when the line is busy, and after hours. Check your system lets you set each one separately, and that you can change them yourself. Our guide to after-hours answering covers the after-hours case.

3. A SIP connection, for the more involved setups

SIP (Session Initiation Protocol) is the standard VoIP systems use to set up calls. Twilio’s Elastic SIP Trunking documentation describes it as connectivity for internet-based phone systems to the public phone network, with origination for incoming calls and termination for outgoing ones. A SIP connection lets an AI platform receive calls from your phone system, and send them back, without the extra step of forwarding.

4. A way to transfer back to a person

The AI should be able to pass a caller to a real person: a direct number, a mobile or a queue. Check the system passes the caller’s number through on transfer, so your staff can see who is calling. If calls are recorded, check the rules where you operate. Our guide to call recording laws in Australia covers Australia.

What each setup does well

A cloud phone system gives a small office extensions, queues, menus and voicemail without hardware to maintain, and staff can answer on a laptop or mobile. Teams Phone suits businesses already working in Microsoft 365.

A SIP trunk gives flexibility and usage-based pricing. Twilio’s United States pricing page says it has no contracts and no channel limits. It suits a business with an existing phone system, or one connecting voice software that needs direct control of calls.

Phone company forwarding needs no new system at all. For a sole trader or a two-person team, forwarding the mobile on no answer to an AI receptionist can be enough.

When a system built and looked after for you fits better

A setup where one team connects the phone system, the AI receptionist, the calendar and the CRM, and looks after them, fits better when calls are a main source of enquiries, several staff answer, and missed calls cost you bookings. Someone has to own the forwarding rules, the transfers and the testing when anything changes.

It does not suit every business. If you get a handful of calls a day, if you or a staff member are happy to set up forwarding yourselves, or if a standard cloud phone plan already does what you need, keep it simple. If you want the AI side built and connected, that is what our AI services cover.

Where to start

1. List every number you use and who holds each account.
2. Decide which calls you want answered by a person, which by voicemail, and which by an AI receptionist.
3. Ask any provider how porting, forwarding rules, SIP connections and transfers work on their plan.
4. Check emergency calling and register your address.
5. Port or forward one number first, test it, then move the rest.