Well, did you check your logfiles for error-messages ?? It is hard to say
what goes wrong without any further information.
Did you try to reset your external device ?? It sometimes happens to me
that my external ISDN-TA freezes and has to be reset.
Again, without Information I can't tell you why the server goes down.
This just happens sometimes due to:
- recognizising bad mood of Admin

)
- it's a weekend and the Admin has a date with someone, the server don't like.

)
+ Not enough Diskspace left for logfiles