How do I get my iPhone to remember which number to use?

When you call one of your contacts, there is no need to select the number to be used each time. Your iPhone uses the same default number that you last used to call this contact. If you have not called this contact before, your iPhone uses your default number. If you wish, you can indicate the number to be used for calling a contact. Take the following steps:

1. Select the contact
2. Select “Preferred Cellular Plan”
3. Select the number to use with this contact