If you want to prevent that number from being able to call your Sony Xperia St23a, you can add a number to your ‘Reject List’.
To add a number to the Reject List , you just need to follow the instructions:
Go to your Sony Xperia St23a
Open the contact page
Select number from the list
Press Menu key
Add to reject list.
Please note that in your Sony Xperia St23a, the Reject List just can block calls from the blocked numbers which are shown with the common blue No symbol
You also can use Third-party apps for blocking phone calls in the Sony Xperia St23a.
This application both blocks calls and SMS which are unwanted, and can help manage a blacklist. It is so easy and lightweight, and does not consume any of your device’s battery power.