Silence on B-Leg, busy tone on A-leg followed by registration fail

[tekstep]

Hello!
We have Freeswitch 1.4.21 x64 and a couple of Yealink T19P,T22,T26P and single T42,T46.
Recently something strange is happening with phones.
Absolutely randomly theres a silence after pickup on callee side and busy tone on the caller side.
Freeswitch log shows NORMAL_TEMPORARY_FAILURE and nothing more.
Immediatly after that the callee phone reports "Registration failed" and starts re-registration which ends successfully. Shortly after re-registration all calls are working normally without any failures.
This issue appears randomly on all our IP-Phones and somehow only on TLS-registered.
Phones registered by UDP are not affected.
Trusted certificates validation is turned off.

Here is a part of T22 syslog from receiving call to hangup.
Could anyone help me with log understanding?

[Yealink_Michael]

hi

thanks for your information

for this issue , please upgrade the firmware of all phone model to the newest firmware that we released, you can download them from below link :
http://support.yealink.com/documentFront...isplayPage

just click according to different models

reset and test again

if the issue still exist , may i know the what is the rate of it ? 1 calls in 10 or 20?

please provide us the trace file for future analysis,trace file including pcap file, config bin and syslog file , and we need three of them provide at one time.

for how to get them , please refer to the FAQ below:
http://forum.yealink.com/forum/showthread.php?tid=1319

after you collect all the files , then please send them to support@yealink.com which is 24 hours work

let me know if any question

TKS & BR

Michael

[tekstep]

All phones have latest non-broadsoft firmware.
Reset have no effect.
Failure rate is almost every call. After registration it works good for 5 minutes or less before the issue reproduced again.
I have sent neccessary files to support@yealink.com