iptables -t nat -I PREROUTING -s 66.192.106.194 -p tcp --dport 5060 -j DNAT --to 66.192.107.225:5061

Shouldn't that rule rewrite the destination port as 5061? Tcpdump says it's not...
19:14:46.528746 IP 66.192.106.194.59240 > 66.192.107.225.5060: . ack 1115 win 64421
19:14:46.917006 IP 66.192.106.194.59235 > 66.192.107.225.5060: F 484:484(0) ack 1115 win 64421
19:14:46.917481 IP 66.192.107.225.5060 > 66.192.106.194.59235: F 1115:1115(0) ack 485 win 6432
19:14:46.917703 IP 66.192.106.194.59235 > 66.192.107.225.5060: . ack 1116 win 64421