I can't guarantee anywhere in your city, but this hack will work pretty much wherever there is cell coverage for the carrier you choose.
You'll want the GSM shield.
Picture from Arduino
Anyway, it uses pins 2, 3, and 7 on it. It also has the ability to solder (on the bottom of it) a microphone and speaker connection. You'll need to buy a SIM card and a contract to connect your shield to a network. I don't know how often you would use this, but I've seen a pay-as-you-go plan that charges you $2 for unlimited talk/text/internet per day. If you're going to use this once a month, that would be the way to go. I didn't look to see if they supported the GSM shield (it was on TMobile's site). You would want to contact them for more details. Also, depending on where you live, I don't know if you would get good coverage. This goes with any cell phone provider, but make sure that you get coverage where you want to go (and don't always trust the maps they provide).