The subject issue will occur when you use connection-oriented callbacks to install the telephony client on Windows XP SP2, because the Windows Firewall blocks the remote procedure call (RPC) port, TCP 251.
To resolve this issue:
1. Start / Run / firewall.cpl / OK.
2. Select the Exceptions tab.
3. Press Add Port.
4. Type RemoteSP into the Name box and 251 into the Port number box.
5. Select the TCP option.
6. Press OK and OK.
NOTE: A restart may be required.
NOTE: If you continue to experience difficulty, contact your ISP.