Common thing on home routers and IP Phones/Soft Phones is that the routers have Application Level Gateways for various services one of which is for SIP. Idea is that the ALG should help in solving any NAT related problems, however in lots of cases the softswitch that your phone is registering to, is a lot better at solving NAT issues than a very cheap in comparison home router, also a lot of the routers SIP ALG’s really mangle up the packets. Try turning off the SIP ALG before you go about trying to punch holes in your firewall.
http://www.voip-info.org/wiki/view/Routers+SIP+ALG
Also for SIP phones if you do end up opening up things in your firewall then there is usually a whole pile of TCP/UDP ports invovled. Quick Google brings up these as common ports for Siemens IP phones.
Port Type Number Service
?UDP 53 DNS PORT
?UDP 3478 STUN SERVER COMMUNICATIONS?
UDP 5060/61 SIP COMMUNICATIONS (plus custom ports)?
UDP 5082 SIP COMMUNICATIONS (OUTBOUND PROXY)
UDP 5004-5020 RTP,RTCP,VOICE