You need to set up a SYS FWD path that goes to other ext then voice mail. Within the SYS FWD path at the original ext click flag for IMMEDIATE. DO NOT INVOKE A MANUAL FWD AT THE ORIGINAL PHONE, only use SYS FWD (phone display shouldn't say FWD). MANUAL FWD won't follow path to vmail. Be sure all call types are included (by default IC is not).
If you need to use this only sometimes, check DND instead of IMMEDIATE and put phone in DND to invoke path.