West Midlands ambulances have had to use mobile phones after vandals cut the service's main radio transmitter cable.
The service said it noticed control room problems and when engineers reached its transmitter site in Dudley they found the cut cable.
The West Midlands Ambulance Service has set up a back up control room but said it is taking longer to get to calls.
"These people do not know the amount of lives they have put at risk," said a West Midlands Ambulance spokesman.
He said it was taking "a little bit longer" to get to incidents because of the communication problems but added the service hoped to have the transmitter fixed within a couple of hours.
"We are getting in touch. The vehicles are all fully aware we have a problem and they are keeping in contact," said the spokesman.
"Hopefully there won't be too many difficulties and we will get to scenes of incidents as we usually do."
West Midlands Police are investigating the break-in.