Problems with Asterisk & Avaya interoperation

[krproperties]

Hello!

We have an Avaya CM 5.1.2 system and an Asterisk PBX connected with h323 trunk.
It works fine, i can call extensions from both sides.

Avaya has E1 uplink from provider to reach PSTN.
I want to call PSTN numbers from Asterisk extensions.

The routing is defined, it works, but there's a problem when calling numbers of one cellular operator, the call is established, but it plays BUSY signal.

When calling another operator, the call comes through fine.
When calling the problematic operator from Avaya's own extension - the call comes throgh fine.

Here are traces from E1 trunk, the first is the one that's made from Avaya's extension and came through:

14:15:56     dial 98xxxxxxxxxx route:ARS
14:15:56     term trunk-group 1    cid 0x95
14:15:56     dial 98xxxxxxxxxx route:ARS
14:15:56     route-pattern  2 preference 1  cid 0x95
14:15:56     seize trunk-group 1 member 30  cid 0x95
14:15:56     Calling Number & Name 495xxxxxxx NO-CPName
14:15:56     Proceed trunk-group 1 member 30  cid 0x95
14:16:01     Alert trunk-group 1 member 30  cid 0x95
14:16:02     idle station    1797 cid 0x95

and another one that originates from Asterisk:

14:16:36     dial 98xxxxxxxxxx route:ARS
14:16:36     route-pattern  2 preference 1  cid 0x97
14:16:36     seize trunk-group 1 member 31  cid 0x97
14:16:36     Setup digits 98xxxxxxxxxx
14:16:36     Calling Number & Name 495xxxxxxx NO-CPName
14:16:36     Proceed trunk-group 1 member 31  cid 0x97
14:16:37     G711A ss:off ps:20 rn:6/1 10.1.0.28:16558 10.1.0.71:16388
14:16:37     xoip: fax:Relay modem:off tty:US 10.1.0.71:16388 uid:0x500a0

It's clear that in the second case the audio path is established and BUSY signal is transferred in-band (from operator?)

What can be the root of the problem?

Thanks in advance