What beeps, the GDS or the phone that is on DND?
What is the phone and GDS connected to?
I assume the DND phone beeps. If this is the case, then it may be related to the DND process.
There are two types of DND implementation - at the PBX and at the phone. If the codes are not synchronized between the phone and the PBX such that both show a DND condition, then only the phone is set to DND. What may be happening is that the INVITE reached the phone and it starts the ring process and then realizes it is on DND and sends a 486 busy message back in reply.
If the PBX is synchronized with the phone such that when the phone is set to DND it also sends a message to the PBX to let it know that the extension is DND, then any INVITE will be stopped at the PBX before reaching the phone and as a result the phone will not peep at all.
Is the GDS actually calling 3 phone or is the GDS really calling a ring group and the PBX is passing the call to the ring group?
The GDS only sends an INVITE to the destination you set and if there is a ring group, queue or other, then the PBX is handling that aspect, not the GDS.