Why not use the iphone with a pay as you go sim (or as a ipod without a sim)? I have been using my iphone with skype as the phone and just a data sim, it works

Why not use the iphone with a pay as you go sim (or as a ipod without a sim)? I have been using my iphone with skype as the phone and just a data sim, it works okay but IP telephony is not as reliable as normal cellular telephony but I can live with it. In my country (denmark) there are no real options for SMS texting without a normal SIM card (no ip based receiving of sms messages) so unfortunately that made the thing a no go for me.