Regarding 'GSM' shields

Do these communicate via cell networks, as in cellphone providers, or just point to point between arduinos? I'm looking to have emergency notifications texted (or other techniques) to a cellphone without the weak-links of needing power company and cable company to be operational.

Where does one go to get service for GSM? (assuming it's available in the US) Does it mean signing up for a pricey cellphone contract with one of the 'bigs' like Sprint, AT&T, etc?

All I see regarding discussing these is in getting them to operate, not the bigger picture of using them.

They act like a normal cell phone when connecting to the GSM network. SIM card and a deal with an operator are needed etc.

Good, I'm on the right track then. Anyone able to recommend a provider in the US that has a low cost data/text-only plan possibly on a per-message basis? It would be silly to have a pricy cellphone package from the usual sources when I anticipate messages over it only rarely as a backup to the hardwired internet connection.