Phne call interference

Compag computer/Pent.2/win98SE/Rockwell 56K fax modem..
Modem works fine,dial up works fine.... No problem with windows,
Here is the problem..Computer is on and idle at desk top..or in standby mode or in Monitor power down mode...computer is not dialed into ISP...the computer shares the only phone line coming into house there is no second or seperate phone line.
Here is what happens the phone rings it is answered by seperate kitchen wall phone a conversation takes place, sometimes when they hang up or at times before they hang up and end the call the computer gets awakened from sleep or disturbed while on desktop not in sleep mode ...either or? and then an error says computer did not shut down properly and does a scan disk.. It does this every time when there is a phone call is this a modem problem?