You should have the option to either create a Lead or a Contact if the system is not finding the email address of the sender in your system.  Or if their email address has changed, you are able to search through available contacts/leads in the “Person” field to find them.
 

Once you’ve created the Contact or Lead, a new button will become available called View Contact (or View Lead) that you can click on to open up the Contact or Lead and add their phone number if that’s what you’re asking. 

@meganfriesen37 I agree with your point. However, our customers are required to enter their contact numbers on the 'Info' page. Can we move the contact number tab there?

If your VAR has someone who can do customizations this might be possible.  Or try adding this as an idea to the portal.

@meganfriesen37  Thank you ...