The dial tone is a flat continuous tone, I can not make any calls.

If you recently dialed 4-0 or 4-1, those are Do Not Disturb and Call Forward codes. if this is what happened, then dial the cancel code for both, which is 4-2. This is with internal dial tone, don't grab an outside to do this. You could have someone dial your extension to test if the ringer is faulty. if their call comes in as voice announce, have them add a 1 to their dial sequence and their call should switch to audible ring, and you will know it's not a ringer issue. Adding a 1 can be done continuously if you want to switch back and forth.

Contact your Phone Carrier.

To set Unconditional Call Forward ON Dial "*72" for a dial tone. Dial the forwarding number followed by "#". Wait for dial tone. LCD will display "Call FWD Activated"
To Cancel Unconditional Call Forward: dial "*73" and get the dial tone, then hang up. LCD will display "Call FWD Activated

but, since you have tried those two, it could be that there might still be a different type of call forward option that is still ON, on your phone like Busy Call Forward and Delayed Call Forward 1. To Cancel Busy Call Forward: dial "*91". Wait for dial tone. Hang up.
2.To Cancel Delayed Call Forward: Dial "*93" for a dial tone, then hang up.

Let me provide you a link to the website which I used as a reference for your future reference.
http://www.grandstream.com/user_manuals/GXP_User_Manual.pdf

Hi, You can use this instruction to activate and de-activate Call divert .Works on all phone


Turning Call Divert On

*
by Touch Tone phone

Step 1 Lift the receiver and listen for the dial tone.
Step 2 Press the Divert On button if using an Optus OneTouch phone, or dial *78 from any other touch tone phone.
Step 3 Listen for three beeps followed by a continuous tone.
Step 4 Dial the telephone number to which the calls are to be diverted.
Step 5 Listen for two short beeps, confirming the diversion.
Step 6 Replace the receiver


Turning Call Divert Off

*
By Touch Tone phone

Step1 Lift the receiver and listen for the dial tone
Step2 Press the Divert Off button if using an Optus OneTouch phone, or dial #78 from any other touch tone phone
Step3 Listen for two short beeps to confirm removal of the diversion
Step4 Replace the receiver.



Other Call Divert Options - Selective Calls
With selective call options, you can choose which numbers you wish to divert, accept or reject by adding these numbers to a list. You can list up to 30 numbers for each call option.

*
Turning On or Off Selective Call Options

Step1 Lift the receiver and listen for the dial tone
Step2 Press the access code for the service you require. You will hear a prompt telling you the status of the service and how many telephone numbers you have on your list. To turn the service on/off press 3.

Access codes Dial
*63 for Selective Call Divert Dial
*68 for Selective Call Accept Dial
*60 for Selective Call Reject
Step3 For Selective Call Divert you will then hear the 'divert to' number that is currently entered. If this number is correct press 1. If it is incorrect, press 0, then re-enter the phone number you wish to have your calls diverted to. When turning this feature off dial 3 again.
Step 4 The feature is now on/off.
Step 5 Replace the receiver.


Note: You can divert calls to a local, national or mobile. However, you cannot divert calls to emergency, 0055, 1900, 008, 1800, 0050, 13, 1300 or international numbers.

Hope it helped.

Thanks for using fixya...

The problem was the main phone line not the phone itself

Have you tried erasing all of your recorded messages? Usually the short dial tone beeps mean you have a pending message.

I just had the same problem. Try unplugging the plug from the electrical outlet for the home(base) unit and plugging back in. Fixed my problem right away.

The only thing I can think of is you may be in the wrong dial mode. If you do not have touch tone service you will hear the tones but it will not connect. Try setting it to pulse and see if you can call out. Check you volume or see if the mute has been set. Have you tried a different phone? you might want to borrow a phone from a neighbor to see if it is a service problem or just your phone.

Refer to your manual. It may be under RX(recieve settings). Go disable FAX/Tel SW and make sure it is fax mode only