Simplest input filter (non-editable country code prefix) for a Phone Number EditText.

CountryCodeInputFilter.java

/**
* InputFilter for Myanmar Phone Numbers
* TODO Update it for all country codes
*/
class CountryCodeInputFilter implements InputFilter {
        
        @Override
        public CharSequence filter(CharSequence source, int start, int end, Spanned dest, int dstart, int dend) {
            //Back Key Press conditions
            boolean deleteCond0 = dstart == 0 && dend == 1; // +
            boolean deleteCond1 = dstart == 1 && dend == 2; // 9
            boolean deleteCond2 = dstart == 2 && dend == 3; // 5

            boolean insertCond0 = dstart == 0 && dend == 0; // ahead of +
            boolean insertCond1 = dstart == 1 && dend == 1; // ahead of 9
            boolean insertCond2 = dstart == 2 && dend == 2; // ahead of 5

            // Handling Back Key Presses
            if ((deleteCond0 || deleteCond1 || deleteCond2) && (source.equals(""))) {
                // BackPress
                if (deleteCond0) {
                    return "+";
                } else if (deleteCond1) {
                    return "9";
                } else {
                    return "5";
                }
            }

            // Handling insertions at invalid positions
            if ((insertCond0 || insertCond1 || insertCond2) && (!source.equals(""))) {
                return "";
            }

            return null;
        }
    }