Chelsie - try this. Go to contacts, open the name in your contact you want to modify. I don't know if it matters where you highlight in the contact but I usually highlight the name or one of the other contact details. Click on options and look for defaults. The mobile should give you the option which numbers for which function you want the default number to be. E.g. you assign the mobile number as the default txt number.
I know you've used the function before but for the benefit of those who do not know. This is useful if you have a contact with several contact numbers (a mobile, landline, number, etc). What this function does is when you create a txt msg and you "add recipient", the default number you chose for the contact will be used without you having to choose from a choice of a few numbers.
Similarly, if you assign another number (a landline number, perhaps) as the voice call default, then when you press the contact name, by default the landline number (or whichever number you assigned) would be called.
Hope this helps.