Do you have the voice encoding within the Link descriptor assignment set to ADI,
Also what version of software do you have on the Mitel and INDeX system's.
I have had many problems with these system linking up together,
However the best setup is to have the Mitel linked to an INDeX with CPU-X installed, otherwise you WILL have problems if you are using the Mitel voicemail on an INDeX phone that has divert to voicemail (Mitel Voicemail)
Hope this helps
Andy