Try Magicall. You can use it to set all kinds of rules for incoming and outgoing calls and messages. In this case, you could set it so that all outgoing calls, or certain contacts get the pbx number and password prepended to them. It runs in the background, so you would just dial the contact normally, and magicall will intercept and dial your pbx, then dial your contact.
The possiblities are almost limitless with magicall, you can set it to only do this with certain contacts, contact groups, or even certain times of day. It really is great software, alot of us have been using it for awhile, and recently started using it for Google Voice dialing.
Here is the link
http://www.mobiion.com/magicall.html