If I understand it correctly, you have 3 remote phones - non-mitel.
You can transfer a call to them from your phone but not your VM?
Look at the COR of your phone. Then assign that same COR to your trunks.
Alternately, (maybe this is the better option) create a system speed dial with the option "over ride toll control" checked and then forward their desk phone to that system speed dial number. This will bypass COR issues.
Ralph