If you are calling the ambulance for yourself, "I've had an accident."
If you are calling it for someone else, "There's been an accident."
But if it really happens, don't worry about it. As long as they hear the word "accident" or "ambulance" they don't care about the grammar. Focus on telling them THE STREET ADDRESS so that they know where to send the ambulance, and answering any questions they ask, (Is the person breathing? Bleeding?), staying on the line, listening for any directions they give you.
Everyone speaks poorly when under stress.