Asterisk + Avaya phones = problems

We have an Asterisk (in form of Elastix) and Avaya 16xx/46xx/96xx phones with SIP firmware.

There’s a problem - when i apply settings in Elastix (after adding an extension, for example) the SIP peers become UNREACHABLE and remain in that state until the phones are rebooted.

Registration timer in asterisk min/max is 60/300, on Phones it is set to 90sec.
But they do not register again on PBX after that period.

Am i missing something?