SIP trunking is a service that allows remote party equipment (typically a PBX installed at customer premisses) to route calls through Multimedia Service Platform.

The PBX has its own SIP accounts and devices that are not known by the MSP. Requests originating from the PBX cannot be therefore authorized based on username/password combinations as they are not provisioned in the MSP subscriber database, they are locally managed by the PBX owner.

The traffic generated by the PBX can be identified by its source IP address(es). To allow traffic from an external PBX, MSP uses the concept of trusted peers. A trusted peer is an IP address that is allowed to route SIP calls through the platform without digest authorization.

Beware that, no checks are done by the proxy related to the incoming caller identity, as long as the SIP sessions originate from the trusted IP address. Once you trust an IP address, you trust all traffic generated by it.

The domain name used by the PBX in the From field must be different than any domain served by the SIP Proxy otherwise the Proxy will challenge the session for credentials as it does for any other locally registered SIP
account.

Outgoing traffic

The IP address(es) of the PBX must be added in the trusted table using SOAP/XML provisioning API.

Caller id indication

MSP itself generates Caller ID indication by appending Remote-Party-Id or P-Asserted identity headers, depending on its configuration, to the outgoing messages routed to the PSTN gateways. Traffic generated by the trusted peers
and any header thereof containing caller id indication is also trusted. When routing calls to PSTN gateways connected to the MSP, make sure that the way caller ID indication is provided by the trusted party is compatible with what the PSTN gateway expects.

Rating

To rate the traffic generated by trusted peers you must add a rating plan in CDRTool rating engine based on the source IP address (the gateway field in rating customers table). Beware that no quota can be imposed on the traffic
of a trusted peer.

Incoming traffic

To route incoming traffic for a number block assigned to the PBX, create ENUM entries that point to the hostname (or IP address) of the PBX. When MSP receives a call form the PSTN for such numbers, it performs an ENUM lookup
and routes the call to the designated PBX.

QoS

MSP is not able to control the amount of sessions going to and from a specific IP address. A PBX has usually limited bandwidth connectivity to the Internet. To maintain an adequate quality of service, a maximum number of
sessions can and should be imposed at the PBX configuration.