- Called and Caller ID normalization:
- Various SIP trunk providers require different number formats (national, e.164, or some special format)
- Different customers vPBX’es may require national number formats for different countries
- Codec normalization: SIP providers are typically limiting the choice of codecs, and customer vPBX’es may require to support a bigger variety of codecs.
- Security: SIP trunk providers may require authorization, and SBC should perform it, screening the global route selection and authorization from back-end systems.
- Admission control: ITSP trunk overload should be avoided
- Where possible, public VoIP should be used for route selection (enum).
- For incoming calls, caller’s name should be looked up if possible (Switzerland).
The following workflow for number normalization is proposed for inbound calls from PSTN:
- Trunk-specific procedure translates the Caller and Called ID into e.164 form
- Called ID is looked up in internal routing database
- Once the internal destination SIP server is selected, the Caller and Called ID are translated into the form required by the destination (typically, national format for a specific country).
For outbound calls originated from back-end servers,
- A special SIP attribute, e.g. X-Dialplan-Country is looked up for a country code (“CH”, “DE”, “FR”, …). If found, the Caller and Called ID are translated from national plan into e.164
- If the country code is not found, the Caller and Called ID are treated as e.164 numbers
- Called ID is looked up in internal routing database. If found, proceed as in the inbound call procedure
- Called ID is looked up in public ENUM database(s). If found, send an unauthenticated SIP call with e.164 numbers in To and From fields
- Called ID is looked up in the local LCR database. If found and if the trunk is not overloaded, the call is sent to a specific SIP trunk. The Called and Caller ID are translated by a trunk-specific procedure to the format expected by ITSP.
- A last-resort ITSP trunk is selected if no other trunks are found.
- ETSI TS 184 011: Requirements and usage of E.164 numbers in NGN and NGCN