Check for valid IMEI

Does anybody know how to check for a valid IMEI?

I have found a function to check on this page: http://www.dotnetfunda.com/articles/article597-imeivalidator-in-vbnet-.aspx

But it returns false for valid IMEI's (f.e. 352972024585360). I can validate them online on this page: http://www.numberingplans.com/?page=analysis&sub=imeinr

What is the correct way(in VB.Net) to check if a given IMEI is valid?

PS: This function from above page must be incorrect in some way:

Public Shared Function isImeiValid(ByVal IMEI As String) As Boolean
    Dim cnt As Integer = 0
    Dim nw As String = String.Empty
    Try
        For Each c As Char In IMEI
            cnt += 1
            If cnt Mod 2 <> 0 Then
                nw += c
            Else
                Dim d As Integer = Integer.Parse(c) * 2 ' Every Second Digit has to be Doubled '
                nw += d.ToString() ' Genegrated a new number with doubled digits '
            End If
        Next
        Dim tot As Integer = 0
        For Each ch As Char In nw.Remove(nw.Length - 1, 1)
            tot += Integer.Parse(ch) ' Adding all digits together '
        Next
        Dim chDigit As Integer = 10 - (tot Mod 10) ' Finding the Check Digit my Finding the Remainder of the sum and subtracting it from 10 '
        If chDigit = Integer.Parse(IMEI(IMEI.Length - 1)) Then ' Checking the Check Digit with the last digit of the Given IMEI code '
            Return True
        Else
            Return False
        End If
    Catch ex As Exception
        Return False
    End Try
End Function

EDIT: This is my working "checkIMEI"-Function:

Public Shared Function checkIMEI(ByRef IMEI As String) As Boolean
    Const allowed As String = "0123456789"

    Dim cleanNumber As New System.Text.StringBuilder
    For i As Int32 = 0 To IMEI.Length - 1
        If (allowed.IndexOf(IMEI.Substring(i, 1)) >= 0) Then
            cleanNumber.Append(IMEI.Substring(i, 1))
        End If
    Next

    If cleanNumber.Length <> 15 Then
        Return False
    Else
        IMEI = cleanNumber.ToString
    End If

    For i As Int32 = cleanNumber.Length + 1 To 16
        cleanNumber.Insert(0, "0")
    Next

    Dim multiplier As Int32, digit As Int32, sum As Int32, total As Int32 = 0
    Dim number As String = cleanNumber.ToString()

    For i As Int32 = 1 To 16
        multiplier = 1 + (i Mod 2)
        digit = Int32.Parse(number.Substring(i - 1, 1))
        sum = digit * multiplier
        If (sum > 9) Then
            sum -= 9
        End If
        total += sum
    Next

    Return (total Mod 10 = 0)
End Function

Solution

IMEI numbers are validated using the Luhn algorithm. The linked page has implementations in various languages. This post also has a few more implementations and a general methodology on how to go about solving the Luhn algorithm.