It happened once both sipgate and ipcomms connected. callee couldn't read CID (neither number nor name). I couldn't explain why it happened.
I think I know what you're talking about. sys.GoogleVoiceCall (GVC) initiated a call using, say, IPKall callback number. Time-out is set to 10 sec... no callback and we issue yet another GVC with SIPGate callback. GVC is waiting for callback and, unexpectedly, we receive it from IPKall. GVC intercepts this callback and you're connected to the callee. Now you receive callback from SIPGate (initiated by 2nd GVC call). nobody's waiting for it and you get it as a regular incoming call ("call waiting").
It happened to me a couple of times, too. If these things happen often, you may need to increase GVC time-out. My time-out is set to 8 sec and I'm quite happy with the results, your mileage may vary.