The East of England Ambulance Service declared a "critical incident" after its 999 call service went down on Wednesday afternoon.
The trust said that its systems failed shortly before 2.30pm, and emergency calls had to be "rerouted to neighbouring services".
Phone lines are now back up and running, and issues with the IT systems have also been fixed.
"We liaised with other blue-light services to ensure they were aware and could contact us by other means," said the trust.
The incident comes just days after the trust urged 999 callers to not hang up as "there may be a delay before we pick up".
The NHS is already under intense pressure as it heads towards winter, with 999 services recording their busiest ever month in October as staff answered a record 1,012,143 calls.
A combination of Covid, winter pressures and recruitment issues are behind the struggles.
Watch an extended interview with Jess Micallef from the union Unison
Jess Micallef, who is the Unison branch Secretary for the East of England Ambulance Service, told ITV News Anglia she had "never seen anything like it."
"In the current climate we won't cope through winter. Winter pressures have not hit yet - when they do we are already at breaking point," she warned.
"The staff themselves within the control room are at breaking point. We have a high level of sickness because people just can't cope."