That's a great question. There are several advantages to having the TA 908e involved in a SIP trunking application between a provider and an IP PBX. Here are a few that we see from other customers:
1. SIP Normalization. With the SIP stack in the TA 900e series, the ADTRAN can act is a mediator between two different vendors implementations of SIP. To overcome interop issues between the PBX and the provider soft switch, the SIP Header Manipulation feature gives you the ability to make specific changes to SIP headers in either direction (towards PBX or the providers soft switch).
2. Survivability. Should the SIP connection to the provider drop, the PRI or FXO ports on the ADTRAN could be used to connect calls to the PSTN.
3. Stateful firewall: The TA 900 has a business class stateful firewall, which includes the ability to handle RTP firewall traversal for VoIP calls.
4. Monitoring point for RTP: Using Voice Quality Monitoring, the ADTRAN can measure jitter, delay, packet loss, and MOS score for RTP streams to and from the PBX/Provider network.
5. Debug point: The ADTRAN can perform packet captures and provide SIP debugs for troubleshooting purposes.
Here are a few additional features that can be used in this application with the SBC Feature Pack:
1. Transcoding: The ADTRAN can provide transcoding for RTP or DTMF between the PBX and the providers gateway.
2. Media Anchoring can be used for topology hiding, or to force media through the ADTRAN if there is already an internet connection used at the customer premise for data traffic.
Hopefully that helps. Let us know if you have any questions regarding any of these features.
Very informative reply, it makes a lot more sense now. Thanks for taking the time to respond.