Hi,
First of all do the need analysis
1. What is the usage requirement of each category of employee (for instances people who needs to contact vendors, need to talk business partners (HR, Admin, Sales, Marketing, Business Development) has more usage so the limit should be different from other supporting functions.
2. See the grade wise requirement
3. Look into voice and voice + data usage (for example, who use heavy attachments and emails may need larger data so now at Rs. 650 per month, you get unlimited 3G data), so simply you may add additional 700 for people using data
4. Lay down the process
5. Define whether the entitlement is automatic or it is case to case basis
I hope this information is useful for you.