Cannot connect to on-hold call

My Setup:

1 Zap channel, 1 IpKall incoming DID, 1 ViaTalk SIP trunk. I use the Viatalk trunk for almost all incoming/outgoing calls.

I have 1 extension hooked to my home telephone wiring through a sunrocket innomedia ATA.

Problem:
Incoming call to viatalk and picked up on one of my home phones. I accidentally press flash and then hangup. I get a ring back for the on-hold call but when I pickup it does not connect to the on-hold call. I just get silence on the phone. Meanwhile the caller keeps on hearing the music-on-hold. I have to go to the Flash operator panel to hang up the call and then have to call back to the caller. I have tried **, *8 etc but cannot connect to the call.

I also have intermittent calls with switching between two callers using flash. sometimes I cannot connect to the second caller and i have to go to FOP to hang up the call.

What am I doing wrong?

Thanks