Why can't I select a phone number to text from within a contact?
With multiple numbers for one contact I am unable to select one to message. It defaults somehow.
iPhone XS, iOS 16
Try this:
Start a new text to the contact. In "New Message" screen To: line press the Circle+ icon. This opens up your "Contacts" Then to be sure of the number you want to use Press and Hold the contact name. A pop up appears and one of the options is "Message". If your contact has more than one number assigned in Contacts to it IE Work, Home, Mobile, your can press the down arrow to the right of "Message" and there you have the option of selecting (Mobile, Work, Home Email Address) for the number you want to use. Press it and that is the number your text message will go to.
From >> Texting to contact with multiple numbers - Apple Community