Depends on if you have Free Incoming Calls.
If yes, only the SingTel Mobile line is chargeable at normal airtime lines (deduct from the call bundle if applicable).
If no, both the SingTel Mobile and StarHub Mobile will be charged at prevailing airtime rates (STM line may deduct from call bundle).
Note that Caller Number Display must be subscribed on both STM and SHM to get the caller's number displayed on screen, not just StarHub Mobile.
Sent from Xiaomi REDMI NOTE 4 using GAGT