What to do if my receptionist is not authorized to accept services on my behalf but did just that?

Question Details: I own a small medical office and have 2 employees - a receptionist and an office manager. The receptionist received a call from the local Yellow Pages for an online advertisement. She said she didn't authorize it but now they are calling everyday and faxing over an invoice and threatening to turn it over to collections. Am I legally obligated to pay this even thought it was "authorized "by my receptionist and not myself? Also, I knew nothing about it until now, apparently this phone call happened 5 months ago.

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