I ran in to an interesting issue a while back where we installed SIP trunks for a customer using an Ingate SIParator and hardware SIP trunks terminating on a ShoreTel SG-90 where when a call would come in to an Auto Attendant at the same site as the SIP trunks but then transfer to another site and the call was not answered the caller would not hear the voice mail greeting. This happens because a ShoreTel 24A is intercepting the call and the voice mail server is not receiving it. You can add the registry entry below to resolve this
HKEY_LOCAL_MACHINE -> Software -> WOW6432node -> ShoreLineTeleworks -> TelephonyManagementServer -> Settings -> switchdebug and add dont_use_Media_proxy 1
