How to connect an AI virtual PBX to VoIP lines

PBX and operator: two different jobs

When people talk about an AI phone system, they think of the voice agent, the menus, the queues. All of that lives in the virtual PBX, which is software. But software is not connected to the telephone network: for a call from a customer's mobile to reach your voice agent, you need a telephone operator that holds the number and delivers the call to the PBX over the SIP protocol.

In this guide the roles are as follows: the PBX is our partner's, the virtual PBX from robottino.ai, with AI voice agent, menus, queues and opening hours; the operator is Vohippo, which supplies the numbers, the SIP lines, the outbound caller ID and the call billing. The same rules also apply to a PBX of your own, such as Asterisk, FreePBX or 3CX.

Keeping the two roles separate has a practical advantage: the number does not depend on the PBX. If one day you change PBX, the number stays with the operator and you only change where the call is delivered.

What the PBX needs from the operator

To work, every PBX needs three things from the operator.

1. One or more numbers that customers call.
2. A SIP line that brings those numbers' calls to the PBX. Usually these are credentials (server, user, password) the PBX uses to register with the operator.
3. A way out for calls that start from the PBX: callbacks, transfers to mobiles, calls made by agents. On this way out, the caller ID shown and the number of simultaneous calls are what count.

In robottino.ai's PBX, each number is added in the phone numbers section of the PBX, entering the number and the operator's SIP parameters. That is where the details Vohippo gives you come into play.

Numbers: which one to choose

The number is the part your customers see, and the choice has practical consequences too.

With Vohippo you will find virtual numbers in 232 Italian cities and more than 90 countries. An Italian geographic number costs €23.48 a month or €109.83 a year, with inbound calls included; it renews automatically from your credit. A geographic number requires documents: for an Italian company, the legal representative's ID, an office in the area and the company registration extract (visura camerale). The outcome usually arrives within 1-3 working days. If you want to understand better how they work, see the guide to the virtual number and how it works.

What if I already have a number with another operator?

Vohippo does not offer number porting, but it does offer «external numbers»: if your number is already on a VoIP line with another operator, the platform keeps it registered using that operator's SIP credentials, for €1.00 a month. The number stays where it is and calls still come into the platform. It is a different solution from porting: the contract with the old operator stays active.

How the call reaches the PBX

With Vohippo there are two ways to deliver a number's calls to the PBX. The choice depends on how the PBX expects to receive the line.

A SIP account registered by the PBX

This is the most common approach with virtual PBXs. In the Vohippo portal you create a SIP account with «Username and password» authentication, and in the number's routing you add a «Ring SIP accounts» step pointing to that account. The PBX registers with those credentials, as a phone would, and receives the calls.

In the PBX you enter details like these:

SIP server / host:   sip.vohippo.com
Port:                5060 (UDP)
SIP user:            the username of the Vohippo SIP account
SIP password:        the account password (12-64 characters)
Registration:        on
Expiry:              300-600 seconds
DTMF:                automatic or RFC 4733

If you use several numbers with different rules in the PBX, the tidiest solution is one SIP account per number: each number in the PBX registers with its own credentials and there is no risk of confusing which number a call came in on.

Sending to a SIP address

If the PBX gives you a SIP address to deliver calls to, you can use the «Send to a SIP address» step in the number's routing, with a sip: URI pointing to the PBX. In this case Vohippo sends the call, without the PBX needing to register. For anyone with their own PBX on site, the classic solution remains the SIP trunk, with password or IP address authentication.

Remember that routing steps can be combined: before sending the call to the PBX you can add, for example, a «Play a message» step with the privacy notice. The full list is in the guide to the call routing modules.

Outbound caller ID

When the PBX makes a call, for example a transfer to the engineer's mobile or a callback to a customer, the operator needs to know which number to show. With Vohippo the number shown must be one of your verified caller IDs: without a valid caller ID the call is rejected and the report shows «Caller ID not allowed». Calls with a withheld number are not possible.

On the SIP account you have two options:

• Fixed caller ID: all the account's calls show the same number. It always wins over the one sent by the PBX. It is the simplest choice if you have only one number.
• «Taken from the call (must be one of your caller IDs)»: the PBX decides the number, call by call. On a password account you can put it in the From field; the format must be +39…, 0039… or national (06…). A number written as 390612345678, without + and without 00, is not recognised.

Numbers bought on Vohippo are already usable caller IDs («Native CLID»). You can also add a number from another operator, verifying it with a code by SMS, WhatsApp or voice call: as an external CLID it costs €0.02 per answered call. All the details are in the guide to caller ID and how to set it. In robottino.ai's PBX the number shown is chosen in the outbound routes, and it can even differ by department.

Simultaneous channels: how many calls at once

A channel is a call in progress. The number of channels decides how many conversations can exist at the same moment on a line. With an AI voice agent this matters more than it seems: the voice agent can talk to several customers at once, so the calls in progress can outnumber the people in the office.

On Vohippo the limit is set per SIP account with the «Simultaneous outbound calls» field, from 1 to 1000 channels; each extra channel costs €0.50 a month, charged pro rata. This limit applies only to outbound calls. For inbound calls, the cap, if you need one, is usually set in the PBX, number by number.

To estimate outbound channels, count the calls that start together at the busiest moment: transfers to mobiles, callbacks, calls made by agents. A transfer to an external number uses an outbound channel for the whole length of the conversation and is charged as an outbound call, according to the per-second rate list.

Audio quality

For an AI voice agent, audio quality is not a cosmetic detail: poor audio makes it mishear names and numbers, and the conversation goes wrong.

• Codecs. Vohippo accounts use G.711 a-law, G.711 µ-law and G.722. G.711 is the codec of traditional telephony and uses about 100 kbit/s in each direction; G.722 offers HD voice where the other end supports it too. Put these codecs at the top of the list in the PBX. Find out more in the guide to VoIP codecs G.711 and G.722.
• DTMF. The keys pressed by the caller (for an IVR menu or a code) travel according to RFC 4733. If the PBX offers a choice, use automatic mode or RFC 4733 events: tones inside the audio are less reliable.
• NAT and firewalls. Between a cloud PBX and the operator there are usually no NAT problems. If, on the other hand, you have phones in the office connected to the PBX, the router matters: turn off SIP ALG and let RTP audio through. The guide on NAT, firewalls and ports explains what to check.

Line security

A SIP line with credentials is a target: whoever steals them can make calls at your expense. Before going live, it is worth setting the following in the Vohippo portal:

• a long, unique password for each SIP account
• the «Daily spending limit», which resets at midnight Rome time
• «Block calls costing more than (per minute)», to avoid premium-rate numbers
• the destinations allowed by country, with «Only the selected destinations» if you only call Italy

If the PBX has a fixed IP address, you can choose «Username/password and IP» authentication: you need both the password and an authorised IP. After 5 failed attempts in 10 minutes the address is blocked automatically; if you block the PBX by mistake, you can unblock it from the «IP address security» page. More advice in the guide to VoIP security.

The checklist before you give the number to customers

1. The number is active in the Vohippo portal and has a routing: a number with no flow gives «Calls rejected».
2. The PBX shows as registered on the SIP account.
3. A test call from a mobile reaches the AI voice agent, and the audio is clear in both directions.
4. A key pressed during the call is recognised.
5. A transfer to a mobile goes through and shows the right caller ID.
6. An out-of-hours call follows the planned route.
7. Spending limit and allowed countries are set.
8. The test calls appear in the call report in the Vohippo portal.

Frequently asked questions