The UK's data watchdog has launched an investigation into a surge of automated calls asking recipients to add unknown numbers to WhatsApp, suspected to be part of a recruitment scam.

The Information Commissioner's Office received more than 3,600 complaints about these robo calls in September, a five-year high and nearly triple the number from July. The ICO suspects millions of people are receiving the calls but filing no report. The automated messages typically come from what appears to be a UK mobile number and often mention part-time work opportunities.

Simon Fell from the Communications Crime Strategy Group, an industry body tackling telecom fraud, said the scam could take multiple forms. He listed possible scenarios: fraudsters could direct victims to fake job listings involving money muling, where criminals pass criminal funds through a victim's account. Alternatively, the calls could be a phishing exercise designed to identify active phone numbers, information that can be resold on the dark web.

The ICO's head of criminal investigations, Andy Curry, told the BBC he believed the calls originated from abroad. He said the 3,600 complaints represent only a small fraction of actual victims. "I think there are probably millions of people who are receiving these messages, which appear to us to be largely indiscriminate," he said.

Fell noted that mobile numbers used in the calls are spoofed, or imitated, by the criminals. He said telecoms companies are responding by sending intelligence reports to regulator Ofcom, working with law enforcement, and liaising with Meta, which owns WhatsApp. Once someone adds a number to WhatsApp's encrypted platform, subsequent conversations cannot be investigated further.

A WhatsApp spokesman said the platform has protective features including the ability to silence calls from unknown numbers. WhatsApp bans millions of accounts annually for "malicious behaviours," including bulk registration and automated messaging.

The UK government committed last November to eliminating call spoofing within a year. A Home Office spokesman said the government was taking "robust action" to tackle scams and remained committed to delivering its Telecommunications Charter by November.

The ICO advises people to ignore calls from unrecognised numbers, block suspicious calls, and report them to the watchdog.