Telephone call based on audio-channel and signalisation-channel.
Signalisation channel based on SIP protocol.
Audio channel based on a codec.
MS uses RT Audio.
Usual SIP-phones use G-codecs. G.711, for example.
You must deploy OCS2.7-based Mediation Server to transcode G.711 <-> RT Audio.
OCS2.7 is looked like SIP-registrar but it is not a true RFC3261-registrar. Say big thanks to MS.