The prank caller once managed to hijack a whole hour of a show, calling in as three different characters, one at a time.
Karlavagnen is one of Swedish Radio's most popular radio programmes, with an average of half a million listeners every weekday evening. It runs on the national P4 station as a late-night chat show, and is based on the calls from listeners, who ring up to tell their stories and share their thoughts on topics either of their own choice or on a theme chosen by the producers.
To get through to the programme, which is broadcast live, listeners have to speak to call handlers, who screen the calls, select the topics and the personalities they feel will contribute to a good programme. As it is public service, they also try to get a good mix of people of different ages, genders and geographical locations.
This week, Swedish Radio programme Verkligheten i P3 (meaning "the reality in P3") revealed that a man who has been calling into Karlavagnen over the past five years has been using different sim cards to get past the call handlers' screening and has pretended to be different characters each time.
In Verkligheten i P3, he calls himself "Kristian" and says he has always enjoyed taking on different roles and pretending to be someone else.
When he has to call companies' phone support for assistance, he uses different dialects and personalities. When he goes abroad, he can tell people that he is an engineer from Germany or Poland, for instance. It is much more fun than just repeating "I'm a Swede and live in Stockholm," he says.
In reality, "Kristian" is in his thirties. He is not an actor or a comedian, but has a creative job, which is partly about creating credible characters. He says he calls Karlavagnen when he is bored, or when he wants a challenge.
"I'll do it again," Kristian says.