Make a short code *601 / Voicemail Collect / "#"201 so you can reach mailbox 201 by dialing *601
Make a phantom user, 601, FWDtoMbx201, and unconditionally forward (including hunt group calls) to *601 (test that dialing 601 reaches mailbox 201)
Make a SEQUENTIAL hunt group, 602, FWD2Mbx201, put x601 in as the only member (test that dialing 602 reaches mailbox 201)
Make hunt group 201 overflow to hunt group 602 after a number of seconds that is LESS than the voicemail answer time for hunt group 200 (test that calling 201 rings and then overflows to mailbox 201)
Call targets HG 200, it is OOS so goes to HG 201 instead, is not answered and overflows to HG 202, with member 601, forwards to *601, which is "#"201 and mailbox 201 answers