Get your Mum to open an account with a US company called Rebtel: http://www.rebtel.com/en/ They will create a local rate phone number for your mobile phone. By calling that number, she can call you using her own landline, or mobile phone regardless of where you are in the world. So even if you're in the middle of the jungle somewhere, or onboard ship, you can both keep in touch with each other even if you don't have access to an Internet cafe.
She pays something like 1 cent a minute to Rebtel plus the price of a local phone call to BT, or whoever her phone company is.
You can make a free test call via the above link to see how it works.
I use them all the time to call friends in Thailand from the Netherlands and the connection is always crystal clear. You don't need a computer to make, or receive calls either.