App and online self service annoying

I have a contract with vodacom which gives me airtime and data every month. I often do not use all my airtime. On the app and online self service there is an option to buy data and airtime bundles for other numbers using your airtime on your phone. I have tried this on numerous occasions when my children have needed data but they ask you to enter your OTP which never ever ever arrives to your phone. I have sent them a message about this but have never got a response. Why have the option to use your airtime to buy other bundles if it never works