A typically drunken phone call made for the sole purpose of sharing the song the caller is currently jamming to with the callee. Most likely the caller and callee have a particular bond over said song.
The act of sending a telejam can have various implications. Most commonly the phone call is made to say "I'm thinking about/missing you". In this case, although the song itself is usually unintelligible due to the extremely high volume it is being played at in conjunction with the caller and caller's drunken friends screeching rendition, this affectionate message is still successfully conveyed. Less commonly the telejam is used mockingly, to rub into the face of the callee how much fun the caller and their other friends are having listening to your special song at 2am on a Tuesday morning.
Either way, the phone call typically ends up as an entertaining voicemail for the callee to listen to at 7am on their way to school or work.
It is standard procedure for Mackenzie, Katie, and April to send one another a telejam whenever intoxicated and listening to Mase's "All I Ever Wanted", paying homage to the Konk days' dorm room music video making and binge drinking of Popov.