How do I add a phone icon button in my navigation?

Site URL: https://ahii.squarespace.com/

Hi everyone, I'm stuck trying to solve this issue.

My client asked for the navigation to be a hamburger at all times AND to add a phone icon button right next to it.

I've managed to do the hamburger and to add a button next to it but I can't convert it into an icon.

Can someone please help me? I'm already past my deadline!

Christophe Bourlon