You won't find what you're looking for in the diag files.
First thing I would try to do is find out which side the Mitel sees the disconnect from.
There is something you can turn on is SMDR that will help.
Here's the text to help you set it up. Note: this does not survive a reboot.
MN00136558 (2K) and MN00136186 (3300ICP) have added an optional indicator "<" or ">" in the first column of SMDR to show which party disconnected a trunk call.
The indicator overrides the long call indicator field (column 1) when enabled.
The indicator works for calls of 5 minutes or less per the dpar.
The command on the SX2000 maintenance terminal on either platform to enable/disable the indicator is:
lldcli smdr_disc [ on | off ].
Do not use brackets or pipes around "on" or "off" (see sample below).
When on, < or > will appear in the 1st position in the SMDR when the call is less than 5 minutes in length.
< indicates we, the pbx, cleared the call
> indicates the far end cleared the call.
This is released in LW34.2.5.2 (34.2.5.3) and 7.0.20.13 (7.0.21.4).
Sample commands & test call from 3300ICP's SX2000 interface:
lldcli smdr_disc on
-----> performed test call
So after you get a complaint of a dropped call then look at the SMDR. If the arrow points to the left then it's the carrier.
Here's my guess on this: The carrier has a T3 that your T1 ends up getting muxed in with. Either the T3 itself is having issues or the card it's connecting to. In the case I would think the T1 drops for a second causing your dropped calls but a check of your T1 will show no problems.
Ralph