Call delivery to an on-premise PBX with dynamic IP address

When a customer purchases DID numbers to associate them with on-premise PBX phone lines, these DIDs are assigned to accounts in PortaBilling.

Depending on the PBX your customer operates, there are several ways to configure delivering incoming calls to PBX:

1. In case a PBX uses a static IP address, calls must be delivered to this IP address. Therefore, you can create accounts (where Account ID is a DID number used to receive incoming calls) and specify the IP address in the SIP contact field. Incoming calls will then be routed to the IP address of the PBX and delivered to the corresponding DID numbers. This way each account (phone line) can have its own configuration (e.g., follow-me lists, voicemail, etc.).
2. In case a PBX uses an IP address that can periodically change (dynamic IP address), the main phone line should be provisioned as the registration account (the account used for registration on the PortaSIP server), and all incoming calls to any of the DIDs are delivered to this account.
3. If a PBX supports multiple registrations, each phone line can be separately registered with its corresponding account on the PortaSIP server. Then an incoming call to any of the DIDs is delivered directly to each account.

Let’s consider the second case when a PBX uses a dynamic IP address. Using call forwarding for incoming call delivery has side effects: it prevents the use of individual forwarding lists for accounts and increases the number of xDRs for a single incoming call.

Therefore, it is desirable to directly deliver all incoming calls to a PBX.

This is done by delivering incoming calls to a PBX with a dynamic IP address via another account (e.g., the registration account). The registration information (IP:port) is taken from the registration account and used by the PortaSIP server to deliver incoming calls to a PBX.

For example, a customer has the DID number 44048882113 and another one, 12065558741, used as a PBX registration account. The PBX is registered on the PortaSIP server with IP address 10.254.203.5. A call to number 44048882113 is delivered to IP address 10.254.203.5.

Step 1. When a PBX registers with account 12065558741, it sends the REGISTER request to the PortaSIP server. This request contains the current registration information (IP:port) in its SIP contact header.

Step 2. The IP:port information is taken from the SIP contact header and stored in the database of phones/CPEs registrations.

Step 3. Someone makes an incoming call to a DID number (44048882113). This call arrives at the PortaSIP server.

Step 4. The PortaSIP server sends an authorization request to PortaBilling. After the authorization checks, PortaBilling returns the authorization response with instructions to the PortaSIP server to deliver this call to account 12065558741.

Step 5. The PortaSIP server takes the IP:port information (from PBX’s registration account 12065558741) and delivers the call to the IP address 10.254.203.5.

Step 6. The phone of account 44048882113 rings.

Whenever an IP address changes, the PBX re-registers with it. Thus, this updated registration information is used for delivering incoming calls to the PBX on all of the accounts.

To specify which PBX account to use for registration, select it on the customer site. All accounts belonging to this customer site inherit the defined settings.

Consider the following example:

A customer has a PBX that uses a dynamic IP address for registration and has 10 DID numbers assigned to it from the range 16045552400 – 16045552409. The administrator configures the PBX to register with registration account 55587412700 and then creates a customer site for the DID numbers and specifies account 55587412700 in the Account field of the SIP Contact service feature.

The PBX receives IP address 145.163.12.123 from the DHCP server. With this address, it registers on the PortaSIP server by sending a registration request with the SIP contact header 55587412700@145.163.12.123:5060. IP address 145.163.12.123 and port 5060 are taken from the header and saved in the database so that when a call arrives to number 16045552401, it is routed to the following destination 16045552401@145.163.12.123:5060.

Later on the PBX is given a new IP address: 20.415.65.4. The PBX re-registers with it and sends the new registration request with SIP contact header 55587412700@20.415.65.4:5060 to the PortaSIP server. Upon successful re-registration, a call arriving at number 16045552401 is now routed to the following destination 16045552401@20.415.65.4:5060.

Then suppose the administrator decides to use the number 16045552404 on a separate SIP phone. He disables the SIP contact feature for account 16045552404 and provisions a SIP phone with this account.

Finally, the PBX is allocated static IP address 155.226.85.170. The administrator defines the registration with the static address and specifies IP address 155.226.85.170 as a host.

Upon registering the PBX, all incoming calls to DIDs are routed to IP address 155.226.85.170:5060.

Thus, delivery of incoming calls via the registration information of an account simplifies the configuration of a PBX with dynamic IP address, significantly decreasing the administrative load.

ITSPs can provide their customers with SIP trunking services regardless of the way the PBX acquires the IP address for registration.