I have a J5780 which I set up in my las property

I have a newer model, and I don't think it locks to a specific fax number, but try this: (By reseting you will lose any stored numbers.)

Here's a link to the manual: http://h10032.www1.hp.com/ctg/Manual/c00844812.pdf

To restore the factory defaults
1. Press Setup.
2. Press 6, then press 4.
This selects Tools and then selects Restore Factory Defaults.
The factory default settings are restored.

To set the date and time
1. Press Setup.
2. Press 6, and then press 3.
This selects Tools and then selects Date and Time.
3. Enter the month, day, and year by pressing the appropriate numbers on the keypad.
Depending on your country/region setting, you might enter the date in a different
order.
4. Enter the hours and minutes.
5. If your time is displayed in the 12-hour format, press 1 for AM, or 2 for PM.
The new date and time settings appear on the display.