I have SIPP as a UAC and then just playing hold music back and echoing the music in SIPP to simulate RTP in both directions.

The only wildcard I can see here is that I am using sub interfaces which don't have:

eth4.2012@eth4: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP

No qdisc settings, not really sure if that changes anything??

Keith