DHCP lease renewal on T21p

[tsukraw]

I know this is not a "Solution" to the issue. But could be a temp fix.
In your DHCP, right click the server, go to Advanced, and set Conflict Detection to 1.

Here is a video if your not sure and want to see how.
https://www.youtube.com/watch?v=VBwVWvxkeBY

What this will do is when a DHCP request comes in, DHCP thinks it has a open IP, DHCP will try to communicate with that IP, as long as there is no response it will give out a lease.
If something communicates back on the IP, DHCP will mark it as in use and go to the next available IP. (This will add no more then a second or two to the DHCP leasing process, nothing a user will notice)

Learned this trick back from the 3CX NBX Phone system days where there phones would not renew DHCP leases after X number of days... Like what you are discribing.

Like i said not a "Fix" but will get you through a bit of the pain.