In my area (north of SF), 911 calls from cellular phones get routed to a call center on the other side of the bay from me (if I recall right). This means that response time for 911 calls from a cellular phone are about 20 minutes, whereas response time for land-line 911 calls is about 5-10 minutes, because my local services can see from the land line my address, and dispatch someone Right Away.

Cardiac Arrest gets significantly more deadly after 7 minutes (I think?), and every second counts. A cellular phone is great in an emergency, but consider using a land line whenever possible.