saidias Chekout.php file in "namedRules" you can edit which fields you want to be required. For the case of the phone you can do something like this
original:
['telephone', 'lang:igniter.cart::default.checkout.label_telephone', ''],
new:
['telephone', 'lang:igniter.cart::default.checkout.label_telephone', 'required|min:6|max:32'],