Skype you can use as a phone if you buy credit. If you have an O2 mobile, download Tu Go and you can use your mobile as normal when connected to wifi. Also if you have apple products might want to try FaceTime too.
Also pretty good is Viber for calls and messaging. Ignore the stickers and other things that make it seem like a teenagers app, the sound quality is actually pretty good. Might also want to look at What's App which I think now allows calls to other users. These are all free to use apps as long as you're connected to a data source.
Most of the cafes, shopping malls and MacDonalds have free Wifi and, unlike UK, you don't have to go through a shite registration process.
So, Skype, etc work fine.
If you are there for any length of time, then PAYG sims (for voice and data) are plentiful - again no onerous address registration or anything. Most of the bigger mobile shops have at least one kid who is fluent in English and can help you get set up (with a sim, that is...).