Windows XP pro

I am not sure if I am connecting everything correctly. I have a dsl modem for ISP. I set up the fax program on my computer to fax from A WINDOWS application. It looks like everything is set correctly but every time I try to fax the fax monitor comes up and says it is not getting a dial tone. I have tried several fax lines to send to but it won’t let it go through. I keep getting the same error.
The external modem for DSL is hooked with a LAN line for my highspeed service and the telephone line is coming from the external modem to the wall connection. This is also a digital phone service. do I need to connect a phone line to the back of my computer?