I would call it a pocket dial. There are certain words that sound faintly ridiculous when said with a British accent and ‘butt’ is one of them.

You might want to look at this thread: https://www.italki.com/discussion/185000 : A Cheeky Question: Have you ever butt dialed anyone? "Butt" ( meaning backside) and "butt dialing" (meaning an accidental phone call) are American terms, but it's not impossible that a British person* might also use them. The phrase we use most commonly in the UK is 'pocket dialling', which is less rude and less funny. * Note that we can't say 'a British'. You have to say 'a British person'. Nationality adjectives ending in 'sh' and 'ch' can't be used as nouns. And while Qasem Mafi's suggestion 'bum call' sounds reasonable (after all, 'bum' is the British equivalent of the American 'butt'), I don't think this is an accepted term. Apart from one reference in The Urban Dictionary - usually a reliable indication of the UNreliability of a definition - I couldn't find any examples of 'bum call'. But who knows, maybe it'll catch on?

It's a butt call. I am British and I wasn't familiar with it a couple of years back and had to ask.

Hi Lucie, The term you are thinking of is "butt dial". Usually this refers to when a call is accidentally made when your phone is in your pocket rather than your bag. Hope this helps! Katy

Bum Call