I have had issues with Cisco 2400 in the past. Apparently Toshiba has seen these issues enough to create a KB bulletin specifically just for theses IADs.

The carrier's field tech will connect a test set to the line and tell you it's the PBX's fault and not their equipment. One of the few times I started to just about started to swear at the carrier's tech was due to problems with a Cisco 2400 to analog lines (I do it behind their backs all the time).

My issue was slightly different. Randomly as soon as we selected a line we got a busy tone from the IAD.

Have the carrier try this:

Change codec from 726 to 711

voice-port 2/0
no supervisory disconnect lcfo
output attenuation 6
no vad
music-threshold -60
caller-id enable

Change voice port settings

voice-port 2/0
no supervisory disconnect lcfo
idle-voltage low
output attenuation 6
music-threshold -60
caller-id alerting ring 2