Call ctr monitoring using VoIP & analog

I am a network administrator in a call center. Our system uses VoIP VIA W98se and an HP server.
The Voice is sent to each station via UDP and the system works fine. I would like to put some kind of server or whatever in the network to be able to serve monitoring phones. This way the supervisors can insure quality on sales calls. I was wondering if anyone else had tried to put some kind of device that would convert UDP packets to analog and deliver that to analog phones? It has to have the capability of allowing the supervisors to punch in a rep station on their cordless phone.

Thanks-