You can use the destination-pattern on your outgoing dial peers to choose which dial peer to use.
If you do not know the number of the 'mobile destination', you will not be able to do this. But, if there is a range of numbers, you can make the destination pattern something like this:
dial-peer voice 1 voip
destination-pattern [5-8]...9....
That would be any 9 digit number starting with 5 through 8 with the 5th number being a 9.
hth,
nick