I'm not sending my phone number by default, so for those I want it to see I have *31# as a prefix. As that number doesn't work for SMS, I usually enter the phone number a second time into Pager.
Unfortunately, I can't switch to Pager (and this is only displayed when in edit mode). How can I choose to display the pager number and use it for SMS?
You can't. You might be better off entering your modified *31# number as the friend's home number, and then using their regular number as the mobile number.
You might also be interested in this feature request at uservoice: http://windowsphone.uservoice.com/forums/101801-feature-suggestions/suggestions/2327547-make-possible-to-send-sms-to-office-and-home-n
