• Call number to confirm a proper fax tone
• Are they having issues receiving faxes in general?
• Do they have another number to try?
• Fax machine issues? (i.e. line issues, jams, out of ink)
• What kind of device is being used to send/receive faxes?
• Are they experiencing faxing related issues with other recipients/senders as well?
• What is the error that is shown on the sender's end when there is a send-failure?