I just bought a Soundpoint 335 for my one line home office. I have comcast voice modem, and a dlink dir-655 router. When I connected the 335 into the router LAN port, the phone initialized, I have a dial tone, but the phone will not register. I also connected directly into the modem, same results. I have adjusted the router's SIP settings, (SIP ALG, enable SIP, etc) to no avail. Will this phone even work over Comcast's voice?
welcome to the Polycom Community.
Registration happens via port 5060
The community's VoIP FAQ contains this post here:
Jan 19, 2012 Question: How to troubleshoot Polycom VoIP related Issues?
Resolution: Please check => here <=
The above post can be utilize to troubleshoot such issues.
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Polycom Global Services
As Steffen said, please try using wireshark to capture the traffic, ideally between the router and the modem, so you can see what's being sent and received, and this will show if the router is mangling the traffic in a way that is causing the registration to fail.
The DIR-655 includes "SPI firewall" functionality, and you may find that disabling this will allow your phone to register.
Alternatively installing OpenWRT on your router (if it is a supported revision) may also help, since the stock D-link firmware can cause issues with SIP registration.