mepr-stripe-customer-extended-args.php

<?php

// Attach a phone number custom field when creating a Stripe Customer object.
// Replace mepr_phone_number with the actual slug of the phone number custom field in MemberPress.
function mepr_stripe_customer_extended_args($args, $user) {
  $phone_number = trim(get_user_meta($user->ID, 'mepr_phone_number', true));

  if(!empty($phone_number)) {
    $args['phone'] = $phone_number;
  }

  return $args;
}

add_filter('mepr_stripe_create_customer_args', 'mepr_stripe_customer_extended_args', 10, 2);