MS Exchange – RAS not working

We have an NT server running SBS 4.5 with SP6 and MS Exchange 5.5. It has been running fine for several weeks.

We did a tape backup today, during the day (ArcserveIT). Before the backup we shut down Norton Anti Virus. We turn it on when done.
Now, when the modem dials…
1st try we get Port already in use Error 633
2nd time we get Remote PPP peer is not responding Error 721

The modem works, but we cannot connect to the ISP. We have verified that the dialup number is correct and is answering.

Can anyone help?