Another way:
Function GetNums(sInp As String) As String
    Dim i As Long
    
    For i = 1 To Len(sInp)
        Select Case Mid(sInp, i, 1)
            Case "0" To "9"
                GetNums = GetNums & Mid(sInp, i, 1)
        End Select
    Next i
End Function