My question is, do you still want the backup number that your SIP provider has to be used in cases where its actually needed? (i assume you intend this number in the event of a total sip failure?)
The situation is that in order for your PBX to play congestion message to your callers, it requires the use of a trunk.
What is potentially possible (taking your trunk capacity into consideration) is to allow 5 calls in, and then the 6th call just play the congestion message for a couple of seconds then issue a hangup. Depending on how your calls come into your PBX this is easy…or cumbersome to configure. Direct dial extensions vs a Queue or ring group.
This COULD sort of service what you are looking for, but in a situation where you have 5 people on a call, then the 6th person calls, and is hearing congestion, if a 7th (or more) person calls in during the time this congestion message is being played, that call would spill to you other line (assuming no change is made)