If you use a phone comany answering service that beeps you when you have messages, Windows NT may detect this as no dial tone.
You can remove the messages from the service prior to dialing or you can open the Modems applet in Control Panel and select the first effected modem. Click on Dialing Properties. Add three commas at the begining of the To access and outside line...... text box. Click OK. Click the Properties button. On the Connection tab, uncheck Wait for dial tone.... Click OK.
Repeat these steps for each modem effected.