Only if you signed up for a subscription before 10/07/2017 will your remaining calling minutes be transferred to mobile data. This happens automatically. Every new month, you'll get 5 extra megabytes of mobile data per unused calling minute. This additional data remains valid for a whole month. Perfect to use your bundle to the fullest!
Please note: this doesn't apply to you if you signed up for a subscription after 10 July 2017.