On the call manager side, don't put the VM ports for MWI dialout in the hunt group used for voice mail ports. This is for 4.x. If on 3.x then don't fwd busy/fwd no answer to the ports used for mwi dial out if you want to dedicate those ports for outdials in unity.
You can make all the ports answering, and put them all in the hunt group, etc. But if all the ports are busy (in use) MWI notification may be delayed as well as other outdial functions such as notifications, transfers to phones.