To the best of my knowledge (which is out of date), audio phone calls from ordinary folk in the UK are not routinely listened to by the UK government. Obviously in the case of an investigation, police could get a warrant to tap a phone of a named person.
What happens with hi-tech communications, now that there is so much voice-recognition, is more open to question and the technology is shifting all the time.
So obviously if you have Siri, you know that it is listening to you. Supposedly if functioning properly it listens but ignores you until you use the wake-word. Obviously there's lots of scope for Siri to be misused in many ways. (My concept of what constitutes misuse may well be the tech companies' idea of normal use...)
This is a really big subject, and I don't want to take over the thread. You'll find quite a lot to read if you look up "surveillance capitalism". Shoshana Zuboff's book The Age of Surveillance Capital has been recommended, but I haven't read it yet.
But in a nutshell, personal data is the new oil.
Article on the book here might be of interest to you:
'The goal is to automate us': welcome to the age of surveillance capitalism
www.theguardian.com/technology/2019/jan/20/shoshana-zuboff-age-of-surveillance-capitalism-google-facebook
“Surveillance capitalism,” she writes, “unilaterally claims human experience as free raw material for translation into behavioural data. Although some of these data are applied to service improvement, the rest are declared as a proprietary behavioural surplus, fed into advanced manufacturing processes known as ‘machine intelligence’, and fabricated into prediction products that anticipate what you will do now, soon, and later. Finally, these prediction products are traded in a new kind of marketplace that I call behavioural futures markets. Surveillance capitalists have grown immensely wealthy from these trading operations, for many companies are willing to lay bets on our future behaviour.”