We have a customer who wants callers to get a busy tone instead of "we cannot connect your call"
This customer has a sip trunk which I have I have set for two simultaneous calls.
Please note that the PBX will send a 486 Busy here to the provider in the event that the maximum lines are reached on the trunk. The prompt you are hearing is not coming from the PBX but from the provider so you should address your request to them.