Alcatel Unleashed

Vad,

The answers are the following:
[1] The external calls are immediately transferred to VM.
[2] The user doesn't hear any warning tones.

Haruthai

Strange. Never hear about such problem.
When incoming DID hold on busy set - 0 - call not camped-on.
When DID overflow on total busy - 0 - PBX not use overflow.

Try to use another Public Network COS, check one more time - user not belong to hunt group.
If problem still exist - use forward on no answer instead of overflow (as temporary solution) and ask your ALU partner about service request.