Multiple SIP links

Hi There,

is it possible to have multiple SIP links to/from different providers?

Currently I have a SIP trunk set-up to a Mitel PBX via Private VPN (pbx on 172.16.0.1) and I now want to make use of external SIP provider, but as soon as the "sip-ua" is changed, our private SIP calls fail.

This is on a 2801 ccme, on a UC500, the calls actually fail if I put in the "correct" ip address under sip-ua

Config is

!
sip-ua
registrar ipv4:172.16.0.1 expires 3600
sip-server ipv4:172.16.0.1
!
!
!
dial-peer voice 2 voip
description Mitel internal Extensions 2**
destination-pattern 2..
voice-class codec 1
session protocol sipv2
session target ipv4:172.16.0.1
dtmf-relay sip-notify
!

Any workarounds available?

thanks