Problem with my hp 640 fax

change from menu the dial tone go to link and see how to change dial tone HP Support document

Make sure the phone line from the wall is plugged into the "line in" jack on your machine

Remove the phone line from the back of the fax and connect a standard phone (not a business PABX or PAX system phone) to the line. Ensure dialtone is available. (Make sure in doing this that you are also testing the cable from the phone socket to the fax). If no dialtone, replace cable or fix line.

Check that the line is connected to the 'line' port on the fax, not the 'phone' port.

If still no dial tone, it may be modem fault (in the fax). Seek specialist attention.

Plug a regular phone into the phone jack and test to be sure you have a dial tone.

Have you set for the proper dialing? there's a choice for pulse or tone dialing, you probably need tone dialing. Also the number needs to be put in just as you would dial on a phone.

Plug a regular phone into the wall jack to verify that you have a dial tone.

Are you plugged into the LINE port?

• If you are using speed dial to dial the fax number, the HP Officejet
might be dialing too fast or too soon. To solve this problem, try one
of the following:
– Try manually dialing the fax number from the HP Officejet front
panel, or from an attached phone. For information, see send a
fax using monitor dialing on page 43 or send a fax manually
from a telephone on page 43.
– Re-create the speed dial setting and insert some pauses. For
information, see create individual speed-dial entries on
page 56.
• If your telephone line has a PBX system, make sure you are dialing
the number for an outside line before dialing the fax number.
• The receiving fax machine might be having problems. To check this,
try calling the fax number from a telephone and listen for fax tones.
If you cannot hear fax tones, the receiving fax might not be turned
on or connected, or a voice mail service could be interfering with
the recipient’s phone line.
http://h10032.www1.hp.com/ctg/Manual/c00095626.pdf

Do you actually have a dial tone on the phone line?
It will still dial without a dial tone.
Plug a phone into the line ans check for the presence of a dial tone. I suspect that to be your problem.