The South Western Ambulance Service has urged people to only call them if it is absolutely necessary ahead of Wednesday's day of strike action.
The service said it expected some staff to strike but it did not know how many.
A spokesperson said it would have to prioritise the most important calls, but an ambulance would be sent to a genuine emergency.
Public sector workers are expected to strike because of changes to pensions and pay.
Dr Andy Smith, the executive medical director at the South Western Ambulance Service NHS Trust said: "If they dial 999 with an emergency that's serious and life threatening they will receive the same kind of response they receive now.
"We will reprioritise our staff to those who really need us," he added.