Google has recently introduced it's new feature, GMail SMS Messaging which is available in so many countries including some asian countries. Using this feature, you can send smss to anyone in any country free of cost.
So if you have zero balance in your mobile and still you want to reply your friends who are messaging you, you have the right option of gmail's sms feature.

Steps To Send SMS Using Gmail:
1. Login to your gmail account and add a new contact of the person you want to send sms to. If you have already that person in the contact list, you can skip this step.
2. Now after adding the new contact, you can see the new contact in your contact list. Now you will have to add the mobile number of that person in his/her contact details. To add the mobile number, click on the Name to edit the details.
3. After clicking the name, a form will appear as shown in the image, you will have to fill the mobile number there and save that.
4. Now, once again go to your contact list and as you hover your mouse pointer on the name of the person, you will see a window as shown in the image. Just clikc on MORE and then click SEND SMS.
5. As you click on Send SMS, you will see the chat box in the right bottom corner of the screen. From there, you can type your sms and can send to your friend free of cost. So, enjoy this service.
Note: You will be given 50 free sms credits. Your one credit is deducted after sending each sms, but if you receive the reply from the person you are sending messages, you will get 5 more credits. So tell your friend to reply to your messages. Standard sms charges will be applied for the person who is replying to the messages.
See the full list of supported countries and operators here.