Multiple Sip listening port

Hi all … i tried gogle but couldnt find anything in the forum …may be didnt use teh correct search words anyways my issue is that i need multipl sip listening ports on the same machine like some of my cleints use 5060 and other need to use alternate ports becuase of isp issues a friend recommended teh following solution .

[code]
Edit /var/lock/subsys/local

iptables -t nat -A PREROUTING -p udp -d 66.xx.xx.xx --dport 5062 -j DNAT --to 66.xx.xx.xx:5060[/code]

Assuming that your sip.conf has port address of 5060 and your ip address is 66.xx.xx.xx

The above command will then allow you to use SIP Client using port 5062 also

This worrked fine and my user cold sucessfully register with the machine but the issue is when they make out going calls it is disconnected exact after 10 sec

the Asterisk CLi shows following

Mar 5 02:24:18] WARNING[2570]: chan_sip.c:1944 retrans_pkt: Maximum retries exceeded on transmission 297a37440841926cMmYxZmVkNTM2NWM3OWQ4ZmVhNzg1OWRjYzg3ODhkNzc. for seqno 2 (Critical Response) [Mar 5 02:24:18] WARNING[2570]: chan_sip.c:1968 retrans_pkt: Hanging up call 297a37440841926cMmYxZmVkNTM2NWM3OWQ4ZmVhNzg1OWRjYzg3ODhkNzc. - no reply to our critical packet.

please suggest methe solution o a better way to have multiple ports open on Sip

thanks inadvance

anyone out there to help ?