Yesterday I tried to decrease both values that you've suggested me for 2 extensions/phones, but after restarting devices, they works for a while and then both stopped working 
So, you're telling that there is nothing wrong with Asterisk version 11.20 and I have the local problem? 
You see, I've already tried to swap non-working/problematic device with the new one, but the issue had arose again, So I really don't know what to do. I also searched for manually downgrade Asterisk version on FreePBX but as I understand, there no official/supported/recommended steps. As official scripts for FreePBX version updates are one way and they can't be rolled back (only restore backed up version of machine). but as we identified the issue too late, I have no idea which backup should I restore.