Hello,
1. please check my suggestion Calling over ISDN, but read also carefully the warnings. 
2. you can use SIP Phones! The genius guys @ snom technology AG, Berlin made it.
They developed the first open standard SIP phone that can register and communicate through OCS directly. (Press Release: May 26th, 2008 )
Please look at http://www.snom.com
or look at youtube for snom ocs edition! http://de.youtube.com/watch?v=Q6CkLVQ2c30
The snom ocs edition is really a limit breaker! The complete 3xx series is now OCS-enabled, simply with a firmware update.
Maybe it will become the "Uber-Tanjay" 
SIPoverTCP, TLS, NTLM, SRTP, etc. - they cut the Gordian knot, without lowering security @OCS !
Best regards,
Jan