T20 Lost registration

Hi,

I have some T20P phones. With Elastix sip server outside the local network. Sometimes when the local lan lost connection with the pbx when the connection returns, the phones says Registered Failed. Is completely random what phones continue registered or not. Only if i change the ip address of the phone returns to register ok. With a reboot the system continues with registered failed.

Someone know this problem?

Thanks a Lot

Jorge