Airtel — Complaint about mnp

Yesterday[protected] i visited airtelstore located at 2 41/2 CHAITANYA CHAMBERS CHAITANYA PURI DILSHUKNAGAR BESIDE HP PETROL BUNK HYDERABAD 500060 . to port my mobile number from vodafone to airtel. the executive in the store simply said we are not porting to airtel prepaid currently.. take postpaid plan and convert into prepaid after 3 months... why should i take postpaid plan.. is there any rule by airtel the mobile numbers which are ported from other networks should take postpaid plan only for the first three months...