Understand that modems connected to networked PC's are a possible "back door" threat to a firewall protected network and need to be addressed as part of the overall network "firewall".
Am sure there are packaged hardware/software solutions to control; including the following:
1. Access control; dial in calls with password authentication (without and with 3rd party security devices).
2. Ability to auto call back after call in.
3. Must be able to dial out from desktop (modem pool) also.
4. Perhaps call logging for audit purposes.
I'm sure this is pretty standard stuff so perhaps someone can suggest a vendor with a range of solutions.