Can't answer 2nd incoming call on 4 line Grandstream 2010

I’m having an intermittent problem with a GXP2010 not being able to pick up a 2nd incoming call when talking to someone on the 1st line.

Most of the time it works but about once a week it quits. I’ve replaced the phone twice and have the latest GS firmware.

The system is with FreePBX 2.7x.

The phone has 2 lines registered with ext 1002 and 1003. Normally when the user is on the phone and a 2nd call comes in line 2 will flash and she just presses the line 2 button to pick up the new incoming call. But about once a week pressing the line 2 button does nothing and she has to go to a different phone to pickup the new, 2nd call. Rebooting the phone seems to fix the problem temporarily.

Does anyone have any advice on how to debug this? I have set sip debug on but I’m not sure how to read the output.

Any advice would be greatly appreciated.