SIP - don't translate source port?

[biskit]

R81.10 Take 55.

We're trying to get a new BT SIP service working.  BT say that the reason it is failing is because all of our SIP traffic comes from random source ports each time.  Packet captures and CP logs absolutely show that we are doing "Xlate (NAT) Source Port" and changing the source port all the time.

BT insist that we turn off SIP ALG and make the firewall keep the source port the same - stop translating the source port.  But how?

We have a custom UDP/5060 port and there is no SIP mentioned anywhere in the fw chain.  So SIP ALG is off.  But how do we stop the firewall from translating the source port?

[PhoneBoy]

Are you performing HIDE NAT on the connection?
If so, the source port changing is expected behavior.
However, this might also apply: https://support.checkpoint.com/results/sk/sk176286