Redirect customer to login page (with other signup plugin) if user not logged in when proceeding to checkout

I use “Digits (WordPress Mobile Number Signup and Login)” for signup by email & SMS ;
when non logged in user proceed to checkout , want to redirect to signup form of Digits (not usual woocommerce signup).

In case already logged-in users, they should go following the next step in the checkout process as normal.

appreciate if anybody help.