Translates a phone number with phoneword letters to a numeric phone number

PhoneWord Translator

public static class Phoneword
{
    /// <summary>
    /// Translates a phonenumber that might contain phoneword letters to the corresponding number
    /// NOTE: Expects a valid phone number
    /// </summary>
    /// <param name="c">The character to translate to the corresponding number</param>
    /// <returns>String with the translated phonenumber</returns>
    public static string Translate(string rawPhoneNumber)
    {
		    StringBuilder newPhoneNumber = new StringBuilder();

        // Prepare number, should be uppercase (eg: 123 JOHN)
        rawPhoneNumber.ToUpperInvariant();
        
        // Loop the phone number string and translate the characters that require translation
        int phoneNumberLength = rawPhoneNumber.Length;
        for(int x = 0; x < phoneNumberLength; x ++)
        {
            // Check if we need to translate the current character
            if(rawPhoneNumber[x] >= 'A' && rawPhoneNumber[x] <= 'Z')
            {
                int nmbr = TranslateLetterToNumber(c);
                newPhoneNumber.Append(nmbr);
            }
            else 
                newPhoneNumber.Append(c);
        }

        return newPhoneNumber.ToString();
    }

    /// <summary>
    /// Translates a Phoneword letter to the corresponding number
    /// </summary>
    /// <param name="c">The character to translate to the corresponding number</param>
    private static int TranslateLetterToNumber(char c)
    {
        switch(c)
        {
            case 'A':
            case 'B':
            case 'C':
                return 2;
            case 'D':
            case 'E':
            case 'F':
                return 3;
            case 'G':
            case 'H':
            case 'I':
                return 4;
            case 'J':
            case 'K':
            case 'L':
                return 5;
            case 'M':
            case 'N':
            case 'O':
                return 6;
            case 'P'
            case 'Q'
            case 'R'
            case 'S':
                return 7;
            case 'T':
            case 'U':
            case 'V':
                return 8;
            case 'W':
            case 'X':
            case 'Y':
            case 'Z':
                return 9;
        }
        return -1;
    }
}