Mailing address for POTS dept?

I have a refund on a cancelled POTS line from August.  It was all setup on line so there is no viable mailing address associated with the account.  I have a small refund of $18 from August.  How do I get a mailing address to request this and give them a mailing address to send it?  I've called different departments and messaged them and get a new number.  Just need a mailing address! Thanks!