In the absence of instructions from a customer, or a disputed bill or a payment arrangement, payments received by the LEC which are insufficient to pay a balance due for both past and current bills shall first be applied to the balance due for past due basic service, then to other past due charges, including late payment charges.
52 Pa. Code § 64.18
This section cited in 52 Pa. Code § 64.24 (relating to provision of bundled service packages).