T46G goes reboot during phone call

[zonder]

Dear Support,

We have 1x MyPBX U500 + 100x T42G + 30x T46G in our university. We just finished installation of IP phones on users desk and started to remove old analog phones.
And then nightmare started...
It is already second week that every day I receive complains from number of IP phone users stating that their phones are accidentally went reboot during calls.
I noticed that all affected persons are T46G model owners (though it could be just coincidence).

Please advice what could be the reason of Yealink IP phones reboot, and how to eliminate this issue ?

Thank you
Here is the part of log from Asterisk PBX, for you interest:
[2014-07-29 12:44:15] WARNING[1814] chan_sip.c: Maximum retries exceeded on transmission 651044870@10.16.0.102 for seqno 2 (Critical Response) -- See doc/sip-retransmit.txt.
[2014-07-29 12:44:15] WARNING[1814] chan_sip.c: Hanging up call 651044870@10.16.0.102 - no reply to our critical packet (see doc/sip-retransmit.txt).
[2014-07-29 12:44:51] NOTICE[1814] chan_sip.c: Peer '2525' is now Reachable. (47ms / 2000ms) IP '10.16.0.102'
[2014-07-29 17:55:14] WARNING[1814] chan_sip.c: Maximum retries exceeded on transmission 1777351062@10.16.0.102 for seqno 2 (Critical Response) -- See doc/sip-retransmit.txt.
[2014-07-29 17:55:14] WARNING[1814] chan_sip.c: Hanging up call 1777351062@10.16.0.102 - no reply to our critical packet (see doc/sip-retransmit.txt).
[2014-07-29 18:01:33] NOTICE[1814] chan_sip.c: Peer '2525' is now Reachable. (23ms / 2000ms) IP '10.16.0.102'