It has come to the attention of OnSIP that there is a bug with how Linksys SPA phones running firmware which is not current do not handle DNS properly.
Typically a phone will attempt to verify the IP address for a SIP Proxy before it registers. The phone does this by doing a DNS Lookup on the service in question and will then send the SIP Registration to that address.
If the IP address of the SIP proxy changes, the Linksys phone will remain unaware of the change and will continue sending the request to the old IP address.
It is highly recommended that if you are using Linksys phones that you update to the latest firmware version to alleviate this issue.
Per the release notes on Linksys's Website, the issue was fixed in 7.4.7 of the Linksys SPA Firmware for SPA50X and SPA30X series. The bug has also been fixed in the Linksys SPA942 with the release of 6.1.5a. Attempts to backtrack and find the release note which addresses the TTL timeout issue for the 942 was not successful but packet captures confirm that 6.1.5a has corrected the problem.
We suspect but cannot confirm that upgrading the firmware to the most recent version should fix the issue for all other Linksys phones.