Thanks for your help so far. Here's what I pulled from ip_conntrack:
udp 17 2893 src=192.168.1.158 dst=18.104.22.168 sport=5060 dport=5061 [UNREPLIED] src=22.214.171.124 dst=70.28.xx.xxx sport=5061 dport=5060 use=1 rate=116 mark=0
The lease time of my IP is a long time, so it's probably not an issue. My IP itself hasn't changed in several months.
Strangely enough, the registration time out has changed:
Aug 16 21:53:30 WARNING: chan_sip.c:9738 handle_response_register: Forbidden - wrong password on authentication for REGISTER for '3xxxxxx9hk' to 's2hkbntel.net'
I know for sure the password is correct since I can authenticate from the ATA, so it's not the problem...