56K Modem Problems

Whenever I turn on my computer and try to connect to the internet, I get one of the following responses.

1. Line is Busy
2. Problems opening port
3. No dial tone detected

This continues to happen until I unplug the phone cord from my computer and plug it back in. Once I do this, I can dial up unto the internet. However, if I get disconnected or log off from the internet and try to log back onto the internet, the problem resufaces. Anyone knows how to fix this?