CALL:
Call 9-1-1 and get an automated external defibrillator ASAP. If you do not have a phone to call, try to send someone to call. If you send someone to call, ensure you send the least experienced person. You always want the most experienced personnel to remain at the scene to provide care.
If you can, use a landline or a public payphone. Although they are getting rare, landlines and public payphones are great because the dispatcher can pinpoint your exact location, even including unit number if you are in a building. Most likely, people who call 9-1-1 will use a cell phone, which is totally fine. But just keep in mind that you will have to give the dispatcher the address and unit number. If you are in the middle of the road, give them the closest intersection with a landmark, such as a restaurant or building.