Ambulance staff who take non-emergency patients to hospital in north Staffordshire are taking industrial action over pay and conditions.
The dispute concerns Parkwood Healthcare, which took over running ambulances from Staffordshire ambulance service last year.
Unison spokesman Ray Salmon said staff will work to rule but are not going on strike at the moment.
Parkwood Healthcare has been unavailable for comment.
"Staff are restricting their duties to the job descriptions," Mr Salmon said.
"They're refusing to use the radios and the mobile phones and they're going back to the depot whenever they need to take instructions.
"Now that's going to incur costs to Parkwood, but it's also going to clog up the system, which will slow people's admissions and discharges from hospitals."