A woman was charged £200 after using Zoom for the first time, she has claimed.
Rachael Lewis said she had a "mini heart attack" when she realised that the money had left her account.
The 34-year-old says she thought that the 50 minute virtual meeting, which took place on January 31, was free.
Two weeks later she received a message from her provider EE explaining that her monthly bill had shot up by £159.63, the Sun reports.
She later discovered that the call was connected via an international number, not data or Wi-Fi, meaning it costs around £4 a minute.
Rachael, from Birmingham, said: "Naturally I had a mini heart attack.
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"My first thought was it must be a mistake, all my calls are unlimited and I don't make overseas calls.
"I am horrified as money is really tight at the moment and it's a substantial amount to cough up unexpectedly."
The mum has vowed not to use Zoom again.
She has said that she couldn't recall being given any warning of premium charges or international call rates.
The charge has left her even more baffled as it was for a union Q&A which was held in the UK.
A spokesperson for EE said that Rachael phoned an American landline rather than used her data, which would have been included in her monthly plan.
She was reportedly offered 25% off as a gesture of goodwill.
A Zoom spokesperson said: "Zoom offers local dial in numbers for over 90 countries and regions, so most users can avoid international roaming charges when choosing to join via phone, rather than using WIFI or data.
"When setting up a meeting involving participants joining by phone, Zoom recommends including the link to local and toll free dial in numbers that is automatically generated when a meeting invite is created.”
The Mirror recently reported how a man nearly fainted when he was told he'd run up nearly £200k in phone charges while on a four-day family holiday in Turkey.
John Nisill, 40, from Skelmersdale in Lancashire, claims he pinged a few emails and spent just 16 minutes on the phone making calls during his trip to Marmaris in October.
But the car salesman was left stunned when his boss called to tell him to switch off his phone because the "bill seems a little high".
John said he nearly fainted when BT told him how much the 43GB of data he'd used would cost.
