One-time use coupons are designed to restrict customers from using a single coupon code multiple times. The system stores the IP address from which the coupon code is submitted and uses that IP address to designate a unique customer.
Administrators are subject to the same one-time use restrictions as customers, so administrators cannot use a one-time use coupon code for more than one phone order, even if orders are created under anonymous users.
If a customer placing a phone order wants to use a one-time use coupon, you’ll need to create a duplicate (or substitute) coupon that does not have the One-Time Use Coupon feature selected. The new administrator-only coupon code can be shared among administrators who use the Phone Order page to avoid the restrictions imposed by one-time use coupons.
For more information, see Coupons / Discounts.