The SMS service Also can be accessed from GoogleTalk and also from the normal Chat in Gmail.
According to Google, the service is supported by all operators in India and the US, but not by all operators in other countries.
How to Send free SMS from your GMAIL account.
The Google SMS help site says: “In the dialog box, enter a phone number in the ‘Send SMS messages to this number’ field.Click here for the list of supported countries and operators. If you are not located in one of these countries you can still use it, but you won’t see the SMS option in Chat until you enable it manually in the Chat settings page.
“A Chat window appears. Just type your message as you would normally. When you hit Enter, the message will be sent to the phone number you entered.
“If your contact replies, the text message response will appear as a reply in Chat. These conversations are stored in your Chat history just like regular chats (but keep in mind that you can’t go off the record while communicating via SMS).”
When the recipient sends a reply, it will be displayed as a chat message.
All users get 50 credits every day. Every time the user sends a SMS, one credit is deducted. If the user receives a reply, five credits are credited to the user’s account, but the total number of credits will not exceed 50.
If you run out of credits, you will get a fresh set of 50 credits the next day, but you can also“buy” credits.
Google Said : “Keep in mind that if you’d like a higher message credit, you can always send an SMS to your own phone, and then reply to that message multiple times. Every time you send a reply message, your SMS credit is increased by five. Effectively, you’re buying more messages by paying your phone company for these outgoing messages.”
“To prevent abuse, any Gmail user who sends an excessive number of SMS messages from Gmail, without getting any responses in return, will be blocked from sending further SMS messages.”