The charity said listening in to calls would help prevent inappropriate relationships from forming after it identified a "very small" number of safeguarding incidents.
A number of "middle-aged men" from a "specific demographic" met up with women for sex after first speaking with them on phone, the Daily Telegraph reported.
Samaritans is a charity that provides support to anyone in emotional distress, struggling to cope, or at risk of suicide.
Its 20,000 volunteers offer support to callers 24-hours per day, 365 days a year. It has taken some 13 million calls since January 2017.
According to the Telegraph, Samaritans branches were made aware of the policy changes last month, with call monitoring set to come into force later this year.
It said the move came after 44 "serious safeguarding incidents" had occurred since 2017.
“I've been a volunteer for six years and I've never seen anything like this,” one volunteer working in the north of England and who did not want to be named, told the paper.
“I felt quite angry that someone could have done it. Obviously, we are speaking to vulnerable people day in and day out. It amazes you that someone could abuse that trust.
“From hearing the talk, I thought it could only be about one of two things - it was either going to be about inappropriate contact over the calls or someone meeting up out of branch. But if there's one thing that’s a big no, it’s that. It’s massive. We’re really well trained but that's the worst thing anyone can do.”
Samaritans said it has robust policies in place for recruiting volunteers, who are bound by strict rules on their contacts with callers.
Julie Bentley, the chief executive of Samaritans, said: “Running any national service on this scale means that, on extremely rare occasions, high standards are not always met and from the millions of calls answered, a very small number of safeguarding incidents were identified.
“Our robust investigation procedures meant these incidents were handled swiftly and appropriate action taken.
“Any safeguarding matter is one too many and as such we review our practices on an ongoing basis and have introduced further measures as part of our commitment to delivering a consistently high-quality experience for our callers.”