A US court has sentenced a Louisiana man to six months in prison after finding him guilty of infecting WebTV users with a Trojan that plagued emergency services with nuisance phone calls.

David Jeansonne, 41, of Metairie, Louisiana, pleaded guilty last month to causing a threat to public safety and causing damage to computers.

He was ordered to pay Microsoft more than $27,100, and US District Judge Ronald M Whyte sentenced Jeansonne to serve an additional six months' home detention.

The WebTV service, which has been rebranded as MSN TV, allows subscribers to connect to the internet using their televisions. Jeansonne's Trojan was emailed to users in 2002, posing as a program which would change colours on their TV screens.

However, the attached file altered the settings on the user's WebTV box making it dial 911 next time it attempted to connect to the internet.

Approximately 20 users are said to have received the email, and 10 reported that the local police either telephoned or visited their home in response to the emergency call.

Graham Cluley, senior technology consultant at Sophos, said: "Making prank phone calls to the police is a dangerous occupation which could have life threatening repercussions for innocent members of the public.

"Writing a program to automatically cause this kind of nuisance demonstrates a whole new level of moronic behaviour."