Just wanted to point out a few facts from the perspective an an active mountain rescue volunteer.
In response to earlier questions that when dialling 911, the phone system will use any available carrier, and it will even work for a phone without a contract. Carriers are required to take all 911 calls they receive. You can bring the tiniest phone possible with no contract and still have the benefit of 911.
The E911 system will give the operator and police/rescue an approximate position based on triangulation of the signal. This is often quite good, and potentially locates a lost person better
than what they can describe to the operator. Just like with GPS if you are in a canyon or near big cliff walls, this doesn't work well due to reflection, but normally you don't have good reception there either.
Probably 95%+ of the rescues we respond to originated with a cell phone call (of the party needing help). But hardly any of them are backpackers... so draw your own conclusions, but
I bring my phone (3.0 oz) off in an ziplock in my first aid kit.