911 is best from a hardwired landline phone (phone jack in the wall). This shows the address associated with that landline and the phone company. Cell phones will go off of GPS tracking and how the signal is hitting the towers...and is not always accurate to the address. I've had my cell phone be a block or two off before...or the community behind mine.
If its an emergency, do not hang up. 911 hangups usually do not get the highest priority of a response. Jiro mentioned it and I will say it too. If you are able to speak at all (even if its not clear), then speak what you can into the phone. Even if its just "Help, Help, Help". This will get the dispatchers attention and help will be coming much faster.
Many 911 call centers have TTY capabilities. Ours does but it is something I have never seen in use.
