SIP Thought Experiment

[Gaff]

This is not really a problem per se, but a minor niggle that cropped up that will probably never be a real-world issue, but someone may be able to help me understand better.

I have a cellphone plan with 100 minutes, so I set up CSipSimple on my Android phone with my VoIPo SIP credentials so I can make calls if I ever exceed this 100 minutes. It works fine, and test calls are placed without issue.

The niggle is that, if I am connected to VoIPo via SIP but attempt to place a normal cellular call to my home VoIPo number, the call is not completed and the home line does not ring. One of two things happens when I try: (i) the call just ends on my cellphone with the message "Call Ended", or (ii) the line rings on my cellphone and after a few seconds a female voice says "Please enter your password followed by the pound key". In either circumstance, the call is not completed and the home VoIPo line does not ring.

I had thought that with SIP in the mix it would just be added as another endpoint for an incoming call, so if someone tried to call while I was connected to SIP, the home line would ring and also the SIP client on my phone would ring. Is this not the case? Does SIP instead "take over" the line whilst it is connected? After the SIP client on my cellphone has been disconnected for a few minutes, the home VoIPo line becomes ringable again.

Just wondering if this is how it was meant to be and my rudimentary understanding was wrong, or what. As I say, it's not really a big problem, just something I was wondering about.