Mobile phone billing issues - iPhone

Good day

I have an account with vodacom with 2 numbers

Main number on flexi contract using with Samsung Android phone Then my second number using with an iphone

For my main number with Android, i can purchase on the play store and do inapp purchases with my airtime through mobile payment.

But for the 2nd number with iphone when i try to add my number for mobile payments, i get an error message.

"There may be a billing issue with your mobile account. Contact your provider or add a new payment method"

I tried by using an android phone for this number by changing the sim card for mobile payment but same error occurs.

Please provide a solution for this as i believe we can do mobile payment with vodacom on both platforms. Dont understand why my second number is failing to do so

Thank you