A new report says Westcountry Ambulance Service staff sometimes feel unsafe and are being put under great pressure to meet national response times.
The Trust has seen a 13% rise in callouts in the last year
The routine report from the Commission for Health Improvement (CHI) says the pressure to meet targets has been at the expense of training, and that staff are being put at risk.
The CHI also praised the highly motivated staff and work done with the local community.
Ambulance driver Nicki Chapman, who is based at Exeter, said a colleague of hers was attacked by a heroin addict.
She said: "I think safety could be improved.
"We have mobile phones on the ambulances, but when you are on location, you are on your own."
Trust chief executive Mike Willis said the report was not entirely accurate.
A lack of stab proof vests for staff
Concerns about vehicle maintenance schedules
Doubts about the level of clinical leadership
"We never intentionally send our staff to a potentially violent incident," said Mr Willis.
"The trouble is that in some cases you cannot always predict that this is going to be the case."
He also said that a survey of staff had shown the vast majority did not want special stab proof vests.
Andrea Groom, assistant director at the CHI, said staff reported passing waiting ambulances while responding to 999 calls.
"The positioning of vehicles is something we would recommend that the Trust looks at and how they use their whole pool of vehicles for emergencies and non emergency work," she said.
"The demands on the service have increased significantly over the last year so they really need to look at how they deploy their vehicles."
The report also highlights concerns about staff and vehicle safety.
It says staff complained about a lack of support from the main control room when they got into difficulties dealing with difficult patients.
The Trust, which is one of the biggest ambulance trusts in the country, has had to contend with a rise of nearly 13% in emergency callouts in the last year.