DNS or DDNS and Yealink phones

[Garfieldone]

I noticed that the T20P and T22P phones have an issue with registering to a hostname if it changes IP addresses. For background we have a number of Cisco 303's, Linksys SPA941,942, SPA 8000, Grandstream HT502 and 503, and a few others. When using a hostname as the SIP server in the phone, it doesn't refresh the IP of the hostname. If I use sample.com and register the phone it will only stay on it until rebooted.

Lets say I need to move the phone from one server to another for maintenance but do not want it visible to the field. I change the DNS for the hostname to another IP. The DNS record is set to refresh every 90 sec. Within a few min all of my Cisco, Linksys, grandstream devices are now registered on the other server. I left a phone for more than 24 hours and it still will not move. The only way to seem to make it do anything is to down the server so that the WatchDog feature reboots it, then it resolves the DNS. I am using static IP's for these phones and for the other brands. Identical settings on all brands. This reacts the same with the older firmware and the new firmware. I also made sure that it is set to register every 60 sec. It does show that it registered 1234@example.com too. I can only suspect that the phone is resolving DNS that first time and never attempts to resolve it after that. How can this be fixed?