🚀 go-pugleaf

 Possibilities (choose or discard according to your situation)

 Cordless phone - carry it with you when you go out to the barn

 A hidden switch to disable the phone.  Or, get two phones.   Rig one up
 to be ring only so you can be aware of incoming calls, and use the 
 hidden switch trick so the other is enabled only when you want to answer
 an incoming call or make an outgoing call.  You can hide the switch most
 anywhere along the wire as long as it is not between the phone company and
 the "ring only" phone.  You could be real tricky and automatically enable
 the regular phone on incoming calls - detect the ring signal and kick in
 a relay.

 These ideas don't limit the outgoing calls to certain numbers, but may
 solve your unauthorized use problem.  They are, of course, worth what 
 you paid for them.

 Mark