Omer,
Must likely it is either the voice codec or T.38 fax. Netmeeting does not support G.729 codec, only G.723 @ 6.3kb, so you must make sure the Tenor is set to G.723/6.3. Additionally, Netmeeting does not follow the H.323 spec correctly for T.38. Even though Netmeeting does not support fax, it should ignore the T.38 (if you have it enabled on the Tenor) capability, but it does not and the calls fail. You will need to disable T.38 on the Tenor to work with Netmeeting.
Mike