Answer
There is no way that FaxMaker or the registry can be edited so that faxes are rejected when all lines/channels are busy sending or receiving faxes.
This request goes against the way the product is designed because the whole point of the queue is for faxes to go through even if the lines are busy.
If you would like to keep lines open for incoming faxes, you can achieve this by configuring lines just for sending and others for receiving.