Notwithstanding the previous answers, someone has already mentioned a VoIP service. To add more details, you can register with a VoIP hosting company, who will provide you with a login to their service.
With software like Zopier you can make outbound calls from your phone. You won't be able to receive inbound calls unless you actually register a number (which will cost for 'line rental') with a VoIP service, and set up call forwarding when you do have cellular signal from your mobile number to that VoIP number.
Regrettably, without a Femtocell there's no alternative to this for text messaging except for iMessage, so unless all of your contacts are on iOS you don't have a choice here.
 there are others - some more here: http://appadvice.com/appguides/show/voip-apps-for-ios