Welcome to the VBA club. It's lots of fun ... after you get used to it
1. REALLY recommend you get a VBA book and read through it and do the examples. I think you have a few fundamental misunderstandings about the VBA language and using macros. A book will provide a much more structured learning environment than asking ad hoc questions.
2. There are VBA functions and worksheet functions. In VBA you were looking for the IsNumeric function which returns something, TRUE or FALSE, so you can act on it (e.g. diferent message boxes). Just 'Call'ing it really won't do much
3. Excel VBA has an 'object' model with things like Worksheets, Ranges, Cells, etc. These as have different properties and methods. Below 'ActiveSheet' is the Worksheet object that is active (usually the one that is selected).
InStr is a VBA function that returns a position of a string within another string, or 0 is it's not there
Option Explicit
Sub bla()
If IsNumeric(3) Then
MsgBox "It's a number"
Else
MsgBox "It's NOT a number"
End If
If IsNumeric("3") Then
MsgBox "It's a number"
Else
MsgBox "It's NOT a number"
End If
If IsNumeric("Three") Then
MsgBox "It's a number"
Else
MsgBox "It's NOT a number"
End If
If IsNumeric(1 + 2) Then
MsgBox "It's a number"
Else
MsgBox "It's NOT a number"
End If
End Sub
Sub Subsegments()
Dim i As Long
For i = 1 To 10000
If InStr(ActiveSheet.Cells(i, 1).Value, ActiveSheet.Cells(i, 2).Value) > 0 Then
ActiveSheet.Cells(i, 3).Value = ActiveSheet.Cells(i, 2).Value
End If
Next i
End Sub