Mumbles Coastguard said it was called in after the phone at Three Cliffs Bay was “abruptly cut off” during a call.
A second call was also made from a phone box claiming a person had fallen from a cliff and died, but it has been “assumed a hoax” with no-one found.
This behaviour has cost thousands,” it said in a Facebook post.
Police, firefighters, an RNLI lifeboat, coastguard crews, lifeguards and ambulance service staff were among those sent to the scene in Swansea at 17:40 BST on Saturday, according to Mumbles Coastguard.
“On arrival the emergency phone was found damaged and out of service,” it said.
“Several services were dispatched and a large search… undertaken with nothing found.
“With no further information or reports, this callout was assumed a hoax call and all assets stood down.”
It added: “This behaviour has cost thousands of both taxpayer and volunteer funds as well has many hours of time on what has been an extremely busy bank holiday weekend.
“It has taken valuable resources away from important taskings, disrupted holidaymakers, local businesses and the lives of our, and our partner services’ families.
“We’d like to thank all those involved with this callout and again express our extreme disappointment in this behaviour.”
Mumbles Coastguard said it had attended four callouts by Saturday afternoon.
The emergency phone is currently out of service.
South Wales Police said it was investigating.
