Source Port/Destination Port for SIP messages

[rime]

When tracing on a Session Manager (putty as "cust" to the security module IP address, then run traceSM, filter on the IP address of the peer with "-i <ip address>"), I see that the Session Manager sends messages with ports widely different from the configured 5061 for that particular signaling connection. Is it predictable which ports will be used? Can't they be nailed down to 5061 or other configured values? I ask because priority on the router is based on IP addresses, protocols and also source and destination ports. Here an interpreted trace extract where the issue is visible:

SESSION MGR............................ PEER
-----------............................-----
Source port: .....0 ....-----INVITE---> Destination Port: 5061
Destination port: 45561 <--- TRYING --- Source Port: .....5061
Destination port: 45561 <--- RINGING -- Source Port: .....5061
Source port: .....0 ....-----PRACK----> Destination Port: 5061
Destination port: 45561 <--- OK ------- Source Port: .....5061