Linksys LRT224 routers might have issues with VoIP service. Here are some settings to be done in the admin interface to correct these issues:

-SIP ALG: Found under firewall settings. Must be disabled.

-SPI Firewall: Found under firewall settings. Must be disabled.

-UDP Timeout: Found under firewall settings. Usually set to 30 seconds by default. Should be increased to at least 300 seconds.

-SIP Transformations: Found under firewall settings. Must be disabled.

-Consistent NAT: Found under firewall settings. Must be enabled.

Persistent NAT connections (system sends NAT keep-alive every 30 seconds)
HTTP over port 80
UDP on ports 5060, 5061 (SIP). Multiple connections must be allowed over these ports.
All internally-initiated UDP connections on ports 10,000-60,000 (RTP).
UDP on port 123 (NTP).


In a broader sense, the phones need to be able to initiate outbound TCP and UDP connections to arbitrary IP addresses on arbitrary ports. Any type of white-list-only configuration on the router will not work.