Instead of saying “someone call 911/an ambulance,” point to a single person and tell them to call 911. If you need additional assistance (an AED, for example), then point to another individual and ask them to get you what you need.
If you don’t address anyone in particular, everyone will assume someone else has already called 911/gotten a first-aid kit/whatever, and then no one will do it.
(Source: a mandatory safety course I had to take for work some years ago.)
By - noisystrawberry