Queue Doesn't Play MOH/Ringtone

Asterisk 1.4.32

Current (Small) Setup:
Inbound Route -> Queue (100) -> Ringall 3 extensions (10/11/12)

My problem, is that when the call from a SIP trunk joins the queue, the line goes silent. This happens regardless of whether it is set to ring or play the default hold music

However: Dialling 100 and joining the queue from one of the handsets, it works fine - the music/ringing works as would be expected.

Does anyone have any thoughts as to what could potentially be wrong?