1.. i want to take 128 unlimited plan ,what speed i will get i used to get 15-16 kbps on sify 128 nightunlimited plan.
You will get about 16 KB/s
2.. is there any downtime
No not really... sometimes I have felt that the connection slows down a little, but not very often.
3.. how many calls r free in phone provided by airtel along with conne.
Depends on the plan you take, I believe the cheapest is around Rs. 250, and you probably get Rs. 200 talktime.
4.. is airtel send huge bills for calls not done
Out of the 9 months that I have had them, there was one month where I thought I had been overcharged - unfortunately you have to pay them Rs. 100 to get a list of the dialled calls (which they would refund if there is a discrepancy) - I think they could just do this for free, especially since they can send it over e-mail at no cost, but oh, well.
5.. is they change there plan monthly
Not sure what you mean but if I understand you, then, no, they don't introduce new plans very often. Also, once you sign up with them for a plan, I don't think they can kick you off it as long as you pay on time.
6.. wheather i get uniform speed throughout day and night.
Yeah, pretty much... not much difference between day and night, which can be a good thing or a bad thing depending on how you look.