Sending texts to contacts with multiple numbers

When I start a new text to a certain friend, I can pop up a list of ways to contact her. The list has three entries: a mobile phone number, a landline, and an email address. The email address has a checkmark by it, which probably means that’s the default. But of course, I don’t want that to be the default. I want her mobile phone to be the default. How do I change that? I tried editing the contact, but I don’t see any way to specify which number to use for text messages.