question: private String normalizeNumber ( String phoneNumber ) { if ( TextUtils . isEmpty ( phoneNumber ) ) { return "" ; } final StringBuilder sb = new StringBuilder ( ) ; final int len = phoneNumber . length ( ) ; for ( int i = 0 ; i < len ; i ++ ) { final char c = phoneNumber . charAt ( i ) ; final int digit = Character . digit ( c , 10 ) ; if ( digit != - 1 ) { sb . append ( digit ) ; } else if ( i == 0 && c == '+' ) { sb . append ( c ) ; } else if ( ( c >= 'a' && c <= 'z' ) || ( c >= 'A' && c <= 'Z' ) ) { return normalizeNumber ( PhoneNumberUtils . convertKeypadLettersToDigits ( phoneNumber ) ) ; } } return sb . toString ( ) ; }
target:  Normalization extracted from Android https://android.googlesource.com/platform/frameworks/base/+/refs/heads/master/telephony * /java/android/telephony/PhoneNumberUtils.java