You can send these SMS on any Network (Pre-paid as well as Post - paid, GSM & CDMA) mentioned below:
Airtel
MTNL - TRUMP
MTNL - Garuda
Vodafone
Tata Indicom
Tata Docomo
Reliance
Idea
BSNL
Aircel
BPL
Spice
Loop
MTS
and
On Any Model of cell phone such as
Nokia
Onida
Videocon
Samsung
LG
Sony Ericsson
These SMS are free, but the operator may charge you the prevailing rates for carrying the SMS in their network !