Have you tried using the digital receptionist ?
Especially the function "Allowing callers to dial a known extension directly" described in the userguide (page 43) is probably offering what you're looking for. If you have a spare voip or DID number you can also have that forwarded to your voicemail extension immediatly.
I think the manual says you need to dial "999" when you get to the Automated Attendant - but, in my case (2 digit extension), it appears to be "99" instead.
I setup a line i could dial with sipgate, then set all incoming calls to divert to 'special menu' and thats the way you can access your VM from outside.