Phones lose connection off and on

My phones go out then a few minutes later they come back on, then go back out. This started after I got a new modem from AT&T, they’ve tested it and said nothing is wrong with the modem, and Internet in general is working fine.

In the logs I see this message a lot:
[2012-07-31 17:47:02] WARNING[21744]: chan_sip.c:3639 retrans_pkt: Retransmission timeout reached on transmission [email protected]:5060 for seqno 102 (Critical Request) – See
Packet timed out after 6400ms with no response

As well as this:
[2012-07-31 11:55:17] WARNING[19229] app_dial.c: Unable to create channel of type ‘SIP’ (cause 20 - Unknown)
[2012-07-31 11:55:17] VERBOSE[19229] app_dial.c: == Everyone is busy/congested at this time (1:0/0/1)

Any ideas what is causing this?

That error indicates that your server can’t talk to the phone. It’s a networking issue, more than likely do to NAT.

There are thousands of threads and articles on SIP/NAT and Asterisk.

Please ask a more specific question.

My gut instinct was its related to NAT.

My asterisk server is on the public internet, and the modem can be a firewall+dhcp server or passthrough to another router.

Should I set the modem to passthrough and then let my linksys router do the NAT translation?