I couldn't come up with much more than simply putting (Default) next to the number, which I'm not too fond of.
Example:
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Sign up to join this communityI couldn't come up with much more than simply putting (Default) next to the number, which I'm not too fond of.
Example:
Primary
You could also use a visual identifier that could move based on which number was the primary number. This visual would need to also provide a textual explanation via a hover of the visual or other means.
You could just ask them to provide a primary contact number and later in the form ask them if the number was a home ,office or mobile number by using radio buttons if you are interested in that information