Unfortunately, this is a decision of Nokia. While Mix Radio is available in other countries, it is unavailable to Asha 501 users in India.
Users of other devices such as Asha 500, 502 aqnd 503 should be able to use it normally.
Just that you can call someone over the internet. Internet Calls are fairly cheaper if you want to call abroad or if your carrier is too expensive (which is rarely in India).
However you'll need to be receiving 3G (UMTS) network and should have a 3G data pack before initiating an Internet Call.
