Two Iranian men in an inflatable dinghy were rescued from the English Channel after the light on their mobile phone alerted a passing ship.
The pair made an emergency call from their 10ft (3m) vessel, and the scant details were passed to Kent Police in the early hours of Thursday morning.
It emerged that the men, who were cold and in a state of shock, had been at sea for up to eight hours before being rescued about a mile (1.6km) off the Port of Dover.
They had no safety equipment, no lifejackets and got into trouble in one of the busiest shipping lanes in the world - the Dover Strait.
RNLI lifeboats from Dover and Dungeness were launched before the master on board P&O's passing Pride of Canterbury ferry spotted a weak light at 3.30am.
When he investigated, it emerged the light was from the stricken pair signalling they needed help, prompting the lifeboats to be diverted to their position, rescuers said.
After being treated on the lifeboat, the pair - one believed to be in his 40s and the other in his 60s - were taken into Dover Harbour where they received further treatment from medics, and Kent Police were waiting.
The rescue comes in the week the National Crime Agency (NCA) revealed that migrants trying to reach the UK are paying smuggling gangs up to £13,500 for their journey.
Some of those intent on coming to Britain are quoted five-figure sums to make the trip by air, while others are even believed to have spent as much as £12,000 to travel from France in inflatable boats.
It also emerged that criminal networks are suspected to have started targeting quieter ports on the east and south coasts in addition to the key hotspot in Kent.
Investigators suspect that, as well as the main Channel crossing between Calais and Kent, criminals may be using less busy ports within the UK, including Tilbury, Purfleet, Hull, Immingham and Newhaven.