Canon MX390 series Online Manual page 18

You can enter the fax/telephone number up to 20 digits, including spaces.
For details on how to enter or delete characters:
Entering Numbers, Letters, and Symbols
6. Enter the unit name.
1. Make sure that Unit name registr. is selected, then press the OK button.
2. Use the Numeric buttons to enter the unit name.
3. Press the OK button.
Note
You can enter the name up to 24 characters, including spaces.
For details on how to enter or delete characters:
Entering Numbers, Letters, and Symbols
7. Press the FAX button to return to the Fax standby screen.