EDIT: (I stand corrected, you can have two mobile numbers per contact)
There are two solutions to this issue.
#1: Add any mobile numbers to the house or work numbers.
When you tap text (SMS) in the contact field. It will by default use the mobile number for the contact.
After you tap text (SMS) at the bottom there are four buttons you can push, the fourth one to the right has two arrows. Select this and from their you can a few options of where you want to send your text too.
First one is in bold which is by default the mobile number. The 2nd and 3rd options are messenger and Facebook messaging.
The last option is "more phone numbers". Once you select this you get the other numbers even if they are work, home or company.
#2. The second option is pretty easy. You can add a new contact with the same name and add a mobile number. From there you can merge the contacts in the people hub.
Hope this helps you.
Sent from my Windows 8 device using Board Express Pro