How can VoIP be integrated into an existing PBX system?

I am trying to integrate VoIP into my existing PBX system. I have a Talkswitch 48CVA which supports both PSTN and VoIP lines.

My main phone number is a landline which forwards calls to my VoIP line when my status is busy or I do not answer. When my first VoIP line is busy/no answer, it is, in turn, set to forward to my second VoIP line. If my landline is in use, the second call will forward to my VoIP line without a problem. However, if my landline and my first VoIP line are both in use, the next call into my landline merely receives a busy signal.

In theory I should not have a busy signal since my first VoIP line should forward to the second VoIP line. The phone company said they only forward one call at a time. If they were all landlines or all VoIP lines I could do hunt/rollover. But since they are a hybrid of both, it seems neither provider wants to accommodate the situation. Do you have any suggestions on how a small business can integrate incoming calls into both landlines and VoIP lines?

If you local provider will not roll two lines then you are kind of stuck. There is a possible work-around though.

Set your VoIP to automatically roll all calls on line one to line two if line two is not busy. That way line one will remain "open" except during the initial roll over. I am not sure if you have that capability in your switch, but it may work.