Make telephone field optional

To make telephone field optional in Checkout: 

  1. from system > configuration > customer > customer configuration > Name and Address Options - set the phone field optional
  2. from Stores > Configuration > Sales > OneStepCheckout > Shipping address fields and settings - set the phone field optional
  3. from Stores > Configuration > Sales > OneStepCheckout > Billing address fields and settings - set the phone field optional