SIP: One way audio

I have the following setup:

 

Setup:

I have SIP server connected to router (WAN). The router is one of the gateways of the BIG-IP LTM. I have a client PC connected to the internal LAN of the BIG-IP.

 

PROBLEM:

When the client REGISTER on the SIP server, the via, contact and to fields shows the internal IP of the client. This causes the server side to be unable to route media back to the client, as the router cannot route to the internal address of the client.

 

How can I ensure that the sent SIP REGISTER message has the correct IP (load balanced IP) in the respective fields, i.e. show the IP of the balanced gateway in the three fields?

 

Steps already taken:

• The SIP virtual server has the SIP profile attached
• I tried to enable the via header and inserted an IP address to see if can REGISTER correctly
• Enabled and disabled source address translation no difference (auto map)
• Enabled and disabled Address Translation (this one worked because the IP address of the BIG-IP as seen by the router is the load balanced one as it should)