Va. Code § 58.1-655
In any return filed under the provisions of this chapter, the communications services provider may credit, against the tax shown to be due on the return, the amount of sales or use tax previously returned and paid on accounts that are owed to the communications services provider and that have been found to be worthless within the period covered by the return. The credit, however, shall not exceed the amount of the uncollected payment determined by treating prior payments on each debt as consisting of the same proportion of payment, sales tax, and other nontaxable charges as in the total debt originally owed to the communications services provider. The amount of accounts for which a credit has been taken that are thereafter in whole or in part paid to the communications services provider shall be included in the first return filed after such collection.
Va. Code § 58.1-655