this is the wrong place to request that. you will need to direct this complaint to digium. Asterisk is a flat resource design with multiple contexts for containers. Some modules support specifying context (like the most recent call parking) while others are entirely per installation (example sip channels, there is nothing in the engine that prevents one sip channel from calling another; or G729 licensing).

If you have a customer doing a lot of faxing then perhaps you should buy more licenses. This license structure is identical to G729. There is no per-tenant control of G729 usage. You could charge per-handset but its always possible to host 4-way calls on a single handset. Its also a similar concept to running a dial-up pool. Maintaining just the right amount of PRI to avoid busy signals while not having too many idle channels is part of the job. If you're trying to keep a 10:1 subscription ratio of fax licenses then you need to start with 10. You'll find that its only in the lower volumes that this ever becomes an issue. Eventually a 10:1 ratio would have so many idle channels it would never be an issue.

you'll also note that the send fax callfile has 12 retries every 300 seconds (5min). So even if all your licenses are in use, its not likely they will be for every single retry. If it is, then you definitely need a LOT more licenses and should consider a different subscription ration because your customer usage is disproportionate to casual use. Inbound calls work similarly, the calling fax station will re-try at a given interval as if the other fax machine was busy receiving a fax.