Problems with telephone T28-P

[vuck]

Regards,
I have it set to 6 SIP lines on the phone YEAHLINK T28P. From the PBX ip randomly telephone calls and for each SIP line button lights up when an incoming call is sent. When there is lot of calls, the 6 buttons each SIP line are turned on simultaneously and the operator captures the call by pressing the appropriate button.
Now my problem: When entering 6 simultaneous calls, only a maximum of 3 buttons light and not displayed the other 3 calls on the phone or the corresponding buttons light to these SIP lines, but before lit up the 6 buttons simultaneously.
I reviewed my PBX IP, and there have been no changes, I think the phone updated its firmware and me is causing this problem. They have heard a similar case ?. I can make a downgrade firmware on the phone? My current version on the phone is:
Firmware version 2.71.0.141
hardware version 1.0.0.53