Cato Berntsen Larsen climbed feet-first into the public facility after his friend dropped the phone while urinating.
Mr Berntsen Larsen said he had volunteered to enter the tank, which is not connected to the sewer and which is only emptied once a season.
The 20-year-old became sick as he stood thigh-deep in its contents, and had to spend an hour completely enclosed in the small area.
