We are having issues trying to pick up a truck from the MBX.
Ha! Funny typo...Pick up a truck. That sound like a mistake my fat fingers would make.
You'll need to have a license for a 3rd party SIP phone on the MBX. Once that is in the system, you'll configure the soft-phone app like you would any other SIP phone. (Usually with some trial and error.) I would try the Bria app, which I think is free.
For the record, I'm not a MBX expert. However, I do know the IPECS which is the MBX's big brother, and the programming is very similar.