[phones]
include => callControl
include => checkVoicemail

[callControl]
exten => preAuth,1,NoOp(well hello there)   ;   <-- this is what gets matched when you do the Goto() from the checkVoicemail context.

[checkVoicemail]
exten => 8500,1,Goto(preAuth,1)

exten => preAuth,1,NoOp(this does not get hit as you might expect)