Can dial out and receive incoming calls but will

Are you connected to a VOIP line for your fax?
A VOIP line can come from a company like Vonage or your local cable co.

If so:

Unfortunatly, faxing over VOIP (Internet) is not always successful. You might want to talk to your VOIP provider if they have a solution.

If not:

Are you trying with the same fax to send and recieve? Some traditional fax machines have probles connecting to fax modems, which is what you have in an Office Jet.
Try having someone else send you a fax and try sending to another number. Otherwise, you might have a defective fax modem.