As a pre-cursor to this move, the numbering plan of the country - which dictates what numbers are reserved for which location/ operator, and how you dial from one to the other - was amended to allow mobile numbers in any telecom circle to be dialled without a 0 or +91 prefix. So when your friend moved to Mumbai without any information, you were still able to dial him without any problem, even though you didn't save his number with 0 or +91 prefix.
When this change happened, many thought it will be the end of STD calling rates, especially when dialling mobiles, since operators will no longer be able to detect the circle of the dialled number just by looking at it. Until now, it's been relatively simple to identify the circle of a number just by looking at the first 5 digits - in fact there are several third-party apps and websites that let you do just that. Once numbers are allowed to move between circles, however, all this goes for a toss.
However, telcos can still identify which numbers belong to another circle. Jatinder Ahuja, Head of India and South West Asia Business, at Amdocs, a leading providers of OSS and BSS solutions to world's leading telcos including ones in India, says that there are additional checks that can be performed to identify if a call is on-network (within an operator's network) or off-network, as well local or STD.
"These checks are performed online at the time of user transaction and includes options such as MNP database queries, location routing number network prefix identification etc," says Ahuja. "The MNP feature in billing software enables telecom operators to perform differential charging for various on-net/ off-net transactions as per their business plans."
An Airtel representative confirmed that numbers that move to another circle are indeed being charged as STD calls to customers in the original circle. This means when your friend moved from Delhi to Mumbai, you started paying STD rates for those calls.
This may not be a big deal for many, especially with the price difference between STD and local calls not being as huge as it used be, but it'll be useful information for at least a few. For instance, maybe you have a pack that gives you free local calling or a certain amount of local minutes, but you find that calls to a particular number that 'looks' local are still being charged to your account balance. Now you know why.