How do you set the default number for your iPhone if there are two mobile numbers under the same contact?

Hello kundan69,

You can set the default phone number for a contact using the process below:

To change the default phone number or email address for a contact method, touch and hold the button for that method below the contact’s name, then tap a selection in the list.

You can also find that information under the "Quickly reach a contact" section of this resource: Add and use contact information on iPhone - Apple Support

Hopefully that addresses your inquiry.

Take care.