I am going to be specifically vague with my response. So if you find that what I wrote doesn't make too much sense, then you really need to either have your Mitel vendor do the work, or have your Mitel vendor sponsor you to take an appropriate Mitel course. You can also request that your Mitel vendor supply you with the system documentation, maybe that will help.
Call forwarding to voicemail is not a function of the voicemail system. The voicemail is at the end of the line, not the begining. So, with that in mind, it is the phone/phone system that controls when a call is forwarded to ANYWHERE. There is a call forward timer in the class of service of the phone, approx 4 seconds per ring cycle. Then you have to program a destination. There can be a system default destination assigned to phones done with Tenanting OR each phone can do call forwarding with either Feature Access Codes or with the Superkey on digital sets.
Do you have a phone that is working the way you want? If so, then copy it's programming parameters to other phones.
Also, if the Multicall ext is not a prime appearance on a physical or virtual circuit, then integration to a specific mailbox will not be possible.
As TheMitelGuy said, we are not here to program your system or to train you on Mitel. Lots of smart, experienced techs here that don't mind helping with very pointed questions provided that you can make use of the answers given.