Unfortunately, there is currently no standard on translating between the two. Conversion is made more difficult by the multiple versions of H.323 (v1, v2, v3). However, there is at least one product (Lucent PacketStar IP) that allows SIP and H.323 terminals to call each other.

There is an ongoing effort within SIP Working Group to define SIP-H.323 interoperation standard. Some details on the effort are available at http://www.softarmor.com/sipwg/teams/sip-h323/index.html. The group has produced an internet dratf that can be found at http://www.cs.columbia.edu/~hgs/sip/drafts/draft-singh-sip-h323-00.txt.