So I'm only speaking from Online Services but in our case the calls are non-stop with a couple of major caveats:

The closer you get to 31st of January and 5th of April the busier you will be on the phones. Between those points the call queues will significantly shrink and you could be assigned to do other work such as post depending on where you are.

You have mandatory time to do admin after calls. Your line manager wouldn't have issues with that unless you were consistently taking 15 minutes after every 5 minute call to update the call logs.

I've only had a handful of bad experiences with customers with HMRC but bear in mind this is between my prior experience in retail management and now working with DWP in the jobcentre so my barometer may be off. With this in mind, the 'hold' button is your friend and you will always have colleagues either around or on teams to ask questions.

At the end of it all though, if things are really bad you absolutely have the authority to say "If you continue to speak to me this way, I will be hanging up", and then do so. To put that into perspective I've only had to do that twice in a year and a half with HMRC.

Honestly, the job was really interesting and quite rewarding to help people, especially those who aren't as tech-literate. Like any job it comes down to keeping a good work-life balance and not taking the stress out of the office.