I see you have a few versions of SL1000 firmware. Are this versions compatible with SL1200 routers?

I have a problem with my SL1200.
The router is changing the IP addresses in the SIP packets and because of this the voice packets are sent to a wrong IP.
Do you know a solution to stop this?
I have incoming calls from an external IP, the inbound rule is configured like ANY, ALL, 5060 NAT to the internal IP.