Hello everyone,
I also have exactly the same problem, its very strange that from 59 Yealink phones the problem has only 9 of them. Thanks to Igaetz for helping me to find out the real reason.
Now I have one more question regarding to it. I checked and found that the bug is fixed 2 days ago on Asterisk site. So, can someone tell when it'll be provided to FreePBX users? As if it'll be soon, then instead of downgrading, it'll better to just wait and upgrade to the newer version. As only couple of phones are affected and after restarting them, they start to work for some time. how do you think?