T21P E2 Downgrade Firmware & PJSIP

[OSPFs]

We have bought two T21P E2 phones with firmware version 52.80.14.3 and when I configure it to work with Asterisk 13, I have found a bug with PJSIP driver. In some conditions we have one-way audio issue with PJSIP, as described here:

http://forum.yealink.com/forum/showthread.php?tid=8330

Then I upgraded one phone to latest firmware 52.82.0.20, but it is not helped me. Furthermore, a new firmware is worse then previous:
Titles on soft keys with russian language are not readeble
LDAP phonebook crashes sometime
And so on

So I have two questions:
Can I downgrade my phone to firmware 52.80.14.3 ?
And is there firmware without PJSIP problem?