Can I know the phone number of the device by code?

First, go to the Android manifest file <uses-permission android:name="android.permission.READ_PHONE_STATE"/> Please allow permission for phone status.

TelephonyManager tMgr = (TelephonyManager)mAppContext.getSystemService(Context.TELEPHONY_SERVICE);
String mPhoneNumber = tMgr.getLine1Number();

It's accessible with these codes.