I've been volunteering for a few years now and trained before that. It is demanding but feels really worthwhile.
In our branch you are asked to do a three hour shift every week and one three hour night duty (between midnight and 6am) each month. No-one enforces this but most people do it.
The training took 9 months (delays due to covid) but is usually about 6. You are very well-supported as you learn to take calls and don't 'go solo' for quite a while.
You can't take the calls in your own home. They are always done at a Samaritans base. (This is good because you can leave the calls behind after your shift and you always have sound support on hand if the call is a difficult one.)
As you might expect, difficult calls can be suicide ones or people unburdening themselves of unbearable suffering . These can be really tough -harrowing even- especially while you are taking them ...but can also be oddly rewarding. Standing with people who feel suicidal or in terrible pain is what we are for. In some calls I have clearly heard how much talking has helped.
Many callers have mental health problems and seem unsupported in the outside world.
Some are struggling with chronic physical pain and need talk for distraction between painkiller doses.
Some have drink/drug problems. Some are gamblers. Some have impossible money or housing troubles.
Many are suffering bereavement or terrible loneliness and need to talk through their pain with someone who can bear it. (Friends and relations often can't.)
We have lots of support afterwards so that we don't end up carrying this sort of thing too long. (I have found so far that even the most upsetting can disappear -like dreams- within a few hours or days.) We have a Samaritans' Samaritan for talking to if we need that but always get to download our shift to someone and get a follow-up next day if it was expecially challenging/distressing.
In response to comments above - yes the other difficult calls are the sex ones. (I've found other Samaritans very supportive to anyone who has suffered such calls. No-one here expects you to be 'tough' about them.) In my experience so far they have always been from male callers. You want to recognise these calls for what they are ... but not misread a genuine caller.
We are free to end sex-calls as fast as we like (because they are revolting/upsetting) or get them onto more serious subjects or to quietly ruin the experience for a the sex-caller. (Who would have thought that being really really boring and unsexy could be a useful talent :) .)