Security firm G4S, which runs Lincolnshire Police's call centre, began investigating in May over claims staff were calling at "quiet times" to improve their response figures.
G4S said two other workers had resigned and a fifth person had returned to work after being cleared of any wrongdoing.
The firm said emergency calls from the public were not affected.
G4S took over the running of backroom services for the force as part of a Â£200m deal in 2012.
The company aims to answer 92% of all 999 calls within 10 seconds, and the firm is subject to financial penalties if performance targets are not achieved.
Two police workers who made fake 999 calls to meet performance targets have been sacked.