Not sure I understand what you're trying to achieve here... If you connect a telephone handset to the extension do you hear dial tone when you lift the receiver?
Answer: Yes
Can you dial out ok on this extension? Yes, with a phone
Is it through a PABX/switch, and do you dial a code to get an "outside line"? Yes
If yes, then maybe you need to programme your modem to dial this code ahead of the number...
The probles is the tone, when i put a headset it works and i can dial, but if I connect the extension to the modem, this it doesn't work, does not give tone