I'm trying to set up a Kamailio server as a proxy to forward SIP messages between two trunks, where I manipulate only the INVITE in the following manner:

incoming call:

INVITE +4969123456@server1

outgoing call:

round robin to

INVITE user@4969123456-sip-server2a

INVITE user@4969123456-sip-server2b

INVITE user@4969123456-sip-server2c

Can anyone point me in the right direction? Both SIP trunks are IP registration only. Also, I am aware how strange this is, but the upstream service has decided to interpret SIP in a novel way.

Cheers,

Chris