Let’s suppose I have a wireless phone with an account in California. While I am on vacation in New York, I snap a picture and send it to my friend back home via text message. Except that my friend is also in New York.
How exactly does the image file go through the network? Is it processed in some central server? Does it ever leave New York?
I believe the standard protocol for this is Multimedia_Messaging_Service. That Wikipedia article answers some of your questions at least.
It mentions a temporary storage server. It isn’t clear to me if it would leave the city.
I believe Apple has iMessage, which is more like an internet based instant messaging style thing, which works from iPhone to iPhone.
Are you talking about the area code and exchange of your phone number is from California?
I am guessing it never leaves New York. Your phone will be transmitting your message to the nearest cell tower, which will send it to a switch center, which would be in the same general area (it doesn’t sent it back to California). From there it needs to figure out how to route it to the destination. Getting to another phone in the same carrier would be straightforward, similar to how a call is routed. I do not know how it works if it has to go to another carrier. But common sense dictates that it is in all carriers’ best interests to provide routing for a given SMS or voice call that the most efficient through the networks, and sending a photo from NYC to California and back again is not efficient.