We have UK mobiles; we are going to Canada on Monday for 9 days & USA for 3 days. What is the best sim / deal to get so it doesn't cost a fortune to receive / make calls to the UK. We can make some on Skype, but will not always be in a WIFI Zone, I need to be contactable 24 hours a day by my mother elderly mother. A new temporary number is fine, but better to use my exisiting one if possible. Thanks!!