A free program for Android, by Steffen Strübe.

The Dual Sim Selector app allows you to manage multiple SIM cards on your Android device. You can configure the app to work with one or more SIM cards, and it will automatically select the correct SIM for outgoing calls. The app supports a variety of rules, such as the number you are calling, your phone number, your current location, and even your phone carrier.

How does it work?

First, install the Dual Sim Selector app on your Android device. Then, create a new rule. Add the phone number you want to select, and set the condition to match the number. Set the priority, and you’re done. The app will now start blocking calls from that number.