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It is not possible to do exactly what you want but you can make something similar. After checking your form I noticed that you already are using Calculation widget and you can reuse that field in order to make logic that will represent something similar you want to achieve.
Here are the steps how you can achieve that.
1. Inside of your Dropdown list you need to set calculation values which will be used in Calculation Widget.
2. Inside of your Calculation widget you need to set calculation logic which will pass No Handphone field value and Dropdown value to calculation widget. In case that you choose option 3 Decimal places in numerical results it will pass first three numbers from No Handphone field. It is not possible to pass last three numbers.
Here is my demo form:
You can clone it in order to see all conditions I made.
Hope this will help. Let us know if you need further assistance.
Considering that your last question is not related to the original question I have moved your last question to the separate thread and we will provide you an answer there: