🚀 go-pugleaf

	Next month East Central Illinois Bell Telephone is going to a local
calling system where you pay about 5 cents for every local call you make.  
The phone company estimates that my bill will just about double.  I have
three teenagers and two lines with call waiting on both lines.  We have had
some trouble in the past with our kids making unauthorized long distance
calls.  One month alone, one kid made $300 worth.  We solved that problem
by having AT+T install a security code for long distance calls, and it has
stopped the abuse.
	My current problem is how to regulate the local calls now that we
must pay for every call.  I want a system where the kid must go to some
central location, put a nickle into a box, and all the phones in the
house will be activated for one and only one call.  At all other times
I still want to be able to receive incomming calls.  This system sounds
a lot like the standard pay telephone at a gas station and we could have
one installed if I can't come up with another option.
	How does the circuitry in a pay phone work?   How can I sence
when the call is over?  How can I still allow incoming calls?  I have a
BS. in EE so a complex circuit is no problem and the labor I put in 
should pay off in the long run.  But this sort of thing has been done 
before and I don't want to re-invent the wheel, so I'm asking for
ideas.   Thanks in advance!