my isp 7star (but now tataindicom) charges prepaid for volume based plansand prepaid/postpaid both plans are available only for unlimited plans.(their website is not working currently). but when i was on 64kb unlimitedthey used to collect money in advance.MTNL is truely postpaid...all plans are postpaid. no disconnection when uexhaust free MBs!
Go for either http://www.beamcablesystem.com/ or BSNL DATAONE, I have been using both for more than a year and without any downtime. Previously I had TATA Indicom that really sucks and they will ensure that will suck every bit of you, Better be away from these bastards. Read all review on Mouthshut.com. But pls pls pls get away from these useless company. BSNL has changed itself qquite a lot and it's very safe, honest and reliable Organisation now. Try it you, typical download speed I get from BSNL is 145 KBPS and it's (2MBPS, basic package)