The Tisbury EMS department has been dipping into its overtime pay budget to ensure full coverage for emergency response, ambulance coordinator Tracy Jones told the town selectmen this week. Ms. Jones said the department is having trouble attracting personnel to fill shifts, and that many shifts are being paid on an hourly basis rather than a stipend in order to meet coverage requirements. The town has two licensed ambulances, one of which must be staffed around the clock seven days a week by a paramedic and an EMT.