can you apply captures on the inside interface of the asa and see if the packets are getting out of the
vpn and whether the cme is responding back,
i dont understand voice technologies a great lot but we with captures we can verify whether the vpn tunnel is th eproblem here or whether it is something el
se
on the asa 5550 apply these captures
access-list capin extended permit ip host 192.168.159.19 host 192.168.107.5
access-list capin extended permit ip host 192.168.107.5 host 192.168.159.19
capture capi interface inside access-list capin
after you initiated some traffic in your case i believe its something to do with registering, you should start capturing traffic and you can view it by
show capture capi
see if you have traffic in both directions or do u see it only in one direction, if you see in one direction it means that there could be some issue with CME
