I asked my wife who spends a large amount of time talking on the phone and she said they are all much the same. I know it is cheaper to call someone on the same network, so perhaps you can do a survey of your friends and find out which network most of them use. With all things considered though, your mobile phone expenses should be the smallest amount of your travel budget. Another option if you have wifi access and your friends have wifi, is to use the "viber" service which is free -
Viber - Free calls, Free text messages, photo and location sharing