Hi Subhamoy
As i checked your code, i found some errors in it.
1. write UBRRL=bd; before UBRRH=(bd>>
; as other wise you will discard lower byte.
2. Write command("ATrn"); instead of command("ATr"); (Enter is required also).
3.
command("ATD+919933988118;r");
_delay_ms(20000);
you cant give more than 6000 ms of delay from your on delay command.
So rewrite 6000 here, for more delay you can write _delay_ms(6000); as many times as you required.
Do these modifications, and tell me the results.