Yeah its a bad situation, because I definitely would like some sort of cap on the fees while not having to pre-pay.
Mum once had a weird situation where she called Boots, hung up (or so she thought), but somehow the call was put on hold and the bill was, well, interesting to say the least. After a lot of effort I did manage to get O2 to waive the cost, but it wasn't fun.
There really needs to be more flexibility to turn off functionality you will never need and an elderly disabled person could accidentally trigger. I don't think it even showed as call active on her phone, Android bug perhaps, it seemed the call only disconnected when she rebooted.
I haven't checked, does the Gigaset say on the other handsets if VoIP is in use? I suppose it might not as VoIP wont have the "line busy" like a landline would.
That's another reason to go with AAISP though, they are more likely to be accommodating with such issues.