It used to be that if a mobile phone customer wanted to switch service providers, they had to give up their old phone number.  And if one had a particularly “good” number or had the number for several years, the inability to retain that number was a huge disincentive to switch services.  But several years ago, the ability to carry one’s number from carrier to carrier became a reality in the U.S., spurring greater competition among carriers.  And with mobile number portability (MNP) being contemplated in India, some consumers there …