It is possible for them to monitor voip calls, if your call passes through their network, or they do remote monitoring on network traffic. The voip traffic is only data that needs to be 'reassembled' to hear the voice stream. However, this is more difficult if the traffic is encrypted between the phone and the call manager or whatever they are using.
I am no legal person, but I am sure that you would have to have agreed to have your calls monitored for them to be able to do this.
Answers & Comments
It is possible for them to monitor voip calls, if your call passes through their network, or they do remote monitoring on network traffic. The voip traffic is only data that needs to be 'reassembled' to hear the voice stream. However, this is more difficult if the traffic is encrypted between the phone and the call manager or whatever they are using.
I am no legal person, but I am sure that you would have to have agreed to have your calls monitored for them to be able to do this.