In order to be able to send and receive fax you need to set the fax baud rate to 9600.  For the HP All-In-One family this works as follows:

From the front panel of the All-in-One :

1. Press * and # at the same time, and then release both keys
2. Press 1, 2, and then 3, releasing each key before pressing the next.
3. The display window should read "Service" with a revision number.
4. Press the right arrow button once.
5. The display should read "System Configuration".
6. Press Enter.
7. Press the right arrow button until "r/w parameter" appears in the display.
8. Press Enter
9. The display should read "parameter 0"
10. Press 153
11. Press Enter
12. The current value will display.
13. Press 2, then press Enter.
14. Press 159
15. Press Enter
16. The current value will display.
17. Press 2, then press Enter.
15. When modifications are complete, press Cancel repeatedly until the date and time appear in the display.