Try to do this, find out if the Tandberg 150 can be set it for voice call, by default all videoconferencing unit makes video call(Tandberg,Polycom) but some H.323 Ip phone can handled voice call only.
Sending content using two or more Polycom 7800 endpoint must works only if the MCU can handled H.239, I do not know if 3511 MCU can handled H.239, may be you just need set a couple of thing in the MCU. A MGC (Polycom's MCU) can use H.329 very well. ...