When using *extension transfer to voicemail, we found the default is for the voicemail greeting to be played immediately, as soon as you dial the extension.
This did not always allow time for the person doing the transfer to hang up, hence the caller might not hear the first few seconds of the voicemail greeting.
We added a 2-second wait which seems to work well - made the following change to extensions.conf:
In macro-vm, insert this new line:
exten => s-DIRECTDIAL,n,Wait(2)
just before this existing line:
exten => s-DIRECTDIAL,n,Voicemail(${ARG1}@${VMCONTEXT}|${VM_DDTYPE}${VMGAIN})