So I checked and verified this "ON HOLD" ATT Service to see if Sprint has something similar. Sprint calls it SEASONAL HOLD. Sprint allows you to cut off service up to 6 mos. when traveling international at $8.99/mo. This is great, because I was assured, if I forget to turn off Data and Roaming or Turn ON Airplane mode, it wouldn't matter since Sprint is cutting service completely. I just got off the Phone with Sprint, and they are unlocking my 4S for International Sim use - will take 3 business days. I was advised that I should only use a 128K sim card when purchasing international sim.
As for restrictions, your only allowed to place 4S, in my case, Seasonal Hold for up to 6 mos. Upon returning back to U.S., you must keep 4S in service for 3 mos. before electing to go back to Seasonal Hold. And since I am still under contract, if I elect to place the 4S on Hold for 3 mos., for example, Sprint simply extends my contract for an additional 3 mos. from the time it was originally qualified for an upgrade.
I wish there was a service by Sprint where I can place the 4S on Seasonal Hold as I travel out of the country 2-3 times a month, but I don't think they will do this. This just gives me added security that I won't get accessed roaming charges when I forget to place the 4S on Airplane Mode or forget to turn Data and Roaming OFF in General Settings.
FYI for those didn't know of this service.