RDP kommunikation mit Client

[Remko]

Just checking that I understand your question correctly:
You have a client pc that start's a Terminal Server Session. From this session you want to initiate something (a TAPI call) on the client and vice versa?
Several solutions possible:

1) Use Virtual Channels, this is a way of communicating between the client and the server with a virtual channel through the RDP (terminal server) or ICA (citrix) protocol. No need to open extra ports or firewall changes.

2) Use named pipes to communicate between client and server.