Regex with numbers and special characters but no letters

I'm making a regex that accepts an input with any decimal(0-9), +, * or # but shouldn't accept any letters(a-z).

so numbers like

• #192#
• *31#+32475728966
• 0479266315
• +32495959511

should be accepted.

The regex is invalid when there is any letter in the string.

• #192#abbef
• a0479266315

This is the regex I have so far:

private const string PhoneNumberRegex = "((\\d)|(\\*)|(\\#)|(\\+))?";

private bool IsValid(inputString)
{
    // Accept * # + and number
    Match match = Regex.Match(inputString, PhoneNumberRegex, RegexOptions.IgnoreCase);
    return match.Success;
}

But this regex also returns true on #192#abbef

How can I fix this?