You may need to ask your applicants for their phone number. The Phone Number field is perfect for this purpose. 

To add a Phone Number field to your submission form, click the Phone Number button on the right side panel of the form builder.

Complete the field label as you normally would.

This is how the field would display to the applicant:

The field also enables the applicant to change the country code by clicking the country dropdown, then choosing the desired country: