T20 and T21p registration problem

[raltel]

I am starting to see this with specific extensions at specific install - Yealink phones (so far just T20 and T21p, latest firmwares) are coming un-registered from Asterisk even though the phone thinks it's still connected - under it's status it shows registered, but Asterisk says no.
I am having the problem with 16 phones, and not having it on the other 35 phones at same location.
The re-register time was reduced to 120 seconds.
All the system work in a dedicated vlan (using HP 1910 POE switchs), installed a year ago, and without configuration or hardware changes on recent time.

Thanks in advance.
Raltel