I was really hoping not to have to do that. because then I have to rematch if something updates. That is why I was looking at -custom.
Specifically this bit is what I need to replicate in order to have their existing functionality work.
[send-to-vm]
exten => s,1,Wait(1)
same => n,SipAddHeader(Diversion:<tel:${EXTDIALED}>\;reason=no-answer\;screen=no\;privacy=off)
same => n,Dial(SIP/Exch4UM5065/6010)
same => n,Hangup()