For years I had only a prepaid SIM(we have free incoming calls paid by caller) and only used wifi data; I saved the prepaid credit, which lasted for 365 days, to send out SMSs when required. I dont know if there is another phone as well suited for being mostly offline as the n900. At the time Evopedia was doing regular offline wikipedia releases and I had scripts to grab favorite websites and podcasts while I had internet in the bus station.