You need a written application given to your area's exchange and they will change it for you beginning with the next month
Just make sure you take a photo ID of the personal under whom the phone is registered and a application signed by the same person
Will take just 10mins to get it done at ur area's exchange
1500 I think is the number you need to call for plan change/ activation. 1504 i think is only for technical support and registering broadband complaints.