Call them up and tell them you didn't know about it, talk sweetly, request and keep at it for a bit, eventually tell them to ask a senior if they don't budge, talk well all this while , and they'll listen.Did it with a friends account and the lady on the other side gave him his 10gb top up back after plan change.Talk nicely, and they'll listen.Also make sure you've cleared your dues.Being an older customer will also help.