Thats because mtnl revised their prepaid 3g data plans weeks ago.So the 2 month Rs 2500 plan is available on MTNL's website but its no longer unlimited and data cap's been set to 100 GB.
ok ok i recharged with 1650/- and got 200 GB,(I am from delhi circle ).
also 2500/- is listed on mtnl mumbai website but not on delhi website.
I am getting 256 kbps to 1.5 mbps speed. variation is too much, may be because of wireless tech.
@ garint you also check the apn in use