Set your phone to use wifi over data when both wifi and data are connected. Most phones do this automatically, I believe. My iPhone 4 does.

Viber using 3g to call from the UK to Ireland
 

Hey I'm using viber on my iphone and its brilliant and love it but i have a few questions about it im going away on holiday to Scotland for 2 weeks and i want to use viber phone calls to call my fiance in ireland and i will only have 3g so if i make a call to him using viber will my phone credit be charged and also what happens if my fiance calls me on viber and we both have 3g will i stil be charged for the phone call

We need to know your carrier. Initially, note that I'm guessing that you are even in Ireland based on your fiancee's location, but this could be an improper assumption. You may be fine using your phone close to normal (with a tad bit of studying). Here is a quick comparison of three Irish providers:

Three

For example, look at Three's rates:

http://apps.three.ie/roaming/pages/d...hree-like-home

Here is data:

http://apps.three.ie/roaming/pages/display/data-roaming

Bottom line, you pay almost the same to use a Three IE SIM in the UK as you do back home. Just make sure your phone remains locked on Three and doesn't wander to someone else.

Meteor

Now let's look at Meteor. It even gets better on voice:

http://www.meteor.ie/do_more/roaming/?linkid=footer

If you're on Meteor, you are better sticking with traditional voice. Note that you need to opt into this plan with the required text.

Vodafone

Now let's look at Vodafone. My quick and dirty study says you are best with Vodafone Passport and sticking to voice:

http://www.vodafone.ie/planscosts/pa...oaming/map/uk/

Both Meteor and Vodafone have ok data roaming packages, but my instinct is just stick with voice.

calls on viber are free, but they use your data, so you will just pay for the 3g data usage, or if you have a wifi connection when you are making/receiving the viber call, it should be free.

-David

Regarding iPhone Apps
 

I love this app as it can also be used in a flying mode if your phone is connected to a wifi.

I like Viber, but it is only good if the other person is on Viber. I have no problem getting my spouse to download an app to chat with me but most people don't. I couldn't get my own brother who simply said he had an unlimited SMS plan to avoid the hassle of this stuff. Two years ago I gifted Whatsapp to my entire extended family virtually no one downloaded it. While I have this stuff, I now just pop for Line2 which gives me unlimited SMS to the US & Canada and $.10 a minute elsewhere together with unlimited calls to the US and Canada. I have ATT's international data plan (unlimited worldwide data plan). I switch to 3g mode only when I live the US, slip on a Mophie case to deal with the added battery suck, and use this approach. Since I cancelled my SMS plan and lowered my minutes, I actually saved money.