Found this service center number for Zong Pakistan.
Go to your SMS settings and add +22676600150 as new SMS
service center number, And activate it if asked on some phones.
Then send totally free SMS messages nationwide or international sms messages.
To send SMS you need to write number in international format, even if
you are sending within Pakistan.
Examples: +9212345678, +9665412345
Enjoy :)
I do not know when this service center number is gonna blocked, leave comments.