Thieves put hundreds of lives at risk by vandalising equipment that left a control centre unable to communicate with its ambulances.
Copper wire was stolen and vital communication wires cut at one of the transmitters used by West Midlands Ambulance Service, in Oakham, near Dudley.
Vandals broke in between 3.30pm and 4.30pm yesterday leaving the service unable to communicate directly with 60 of its ambulances for several hours.
Trevor Ford, spokesperson for West Midlands Ambulances Service, said: "This mindless incident put many peoples lives at risk.
"We were still able to take calls but we could not get in contact with our ambulances as quickly as we normally would."
Communication was restricted to back-up mobile phones for several hours, causing major delays in response times.
However, the service could not properly track their vehicles and quickly establish which was the closest to respond to emergency calls.
Engineers and riggers were immediately called to fix the problem, trying to replace the cable as quickly as possible.
In addition to cutting one of the main cables, thieves also stole copper wiring from the transmitter.
West Midlands Police is investing and it is not known if the transmitter is covered by any CCTV cameras.