Be it any service provider, only difference at the end of the month shall be how much talk time you used, and you are to pay for it.
Lets take a look at all the services individually.
Reliance is coming with Rs.501.00 down payment and rest in Rs.400.00 monthly payment for 3 years. It has Rs.499.00 monthly commitment, but gives Rs.400.00 worth of talk time free every month. This is all on papers, but when you read between the lines, you shall find that the talk time shall not be in actual value, it does not imply on all the calls, some calls are far costly in comparison to land lines.
It also promises Rs.0.40 per call. But the glitch is it means only R2R(reliance to reliance) calls.
You can take a look at all the other services together, And they are in reality, having one tariff, one new scheme from Idea shall spur similar(different by just 5 rupees) schemes from Airtel, or any other service providers.
One more thing, there are two basic type of services, postpaid and prepaid. In post paid one can use the telephone services and pay at the end of the month. In prepaid one have to keep some amount in his/her account to use to telephone, using rechargable cards which are very easily available everywhere, thanx to wonderful market network.
You need to weigh your pro and cons before choosing any scheme/plan. Check your phone diary, which no. will you be calling most, land line? or cellular? are there some No. which would be in use very very frequently? is it ISD that you are looking for? or is it just incoming you want, leaving minor outgoing calls?
There are plans/schemes for allthe requirements by different companies, e.g. BSNL have thrown out a unique plan in the market for only incoming calls(unlimited) for Rs.169.00 monthly rental. Airtel have thrown Rs.150.00 monthly rental plan for post paid services countering Rs.169.00 plan of Idea.
There are plans/schemes for cheaper C2C(cellular to cellular) calls, but costly C2L(cellular to landline), and exactly mirror to this also.
There are schemes with higher monthly rental(Rs.1700.00) but very cheap call-rates(e.g. Idea 1699 buzz plan: monthly rental Rs.1699.0, incoming free, outgoing Rs.0.40 per min for intracircle, and Rs.1.99 for STD)(Intracircle generally means within the state of origine of connection)