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SIP call, can't send RTP on bound UDP port after sending ICMP packet


We have a strange issue with a SIP call where if we receive even a single RTP packet whilst the port is closed before we have sent a keepalive packet to open the pinhole, any packets subsequently sent come from src port 1042. The far end isn't expecting that so it results in no audio.

However if we managed to send the keepalive before receiving RTP from the far end everything works fine and we have audio. We receive an RTP packet destined for port 30322 but we the port isn't open so an ICMP packet is returned. However when we then try to send the keepalive it comes from port 1042 rather than 30322. This is after the second 200OK on the second leg of the call. Anyone know why this is happening. We can send a Wireshark trace if required, we would have uploaded but the system isn't allowing.