This is completely normal. Your bank will block the full amount of the phone on your credit card limit when you purchase your Phone Combo Plan with your phone cost payable over 24 months.
This blocked amount is reduced every month as you make the monthly installment payment for your phone.
Similarly for existing customers, if you chose the 24 month installment payment option for your phone purchase, the full cost of your chosen phone will be blocked on your credit card by your bank.