Making VBA Form TextBox accept Numbers only (including +, - and .)

use the KeyPress event, and discard any non-numeric entry:

Private Sub txtShift1_KeyPress(ByVal KeyAscii As MSForms.ReturnInteger)
Debug.Print KeyAscii
If KeyAscii >= 48 And KeyAscii <= 57 Then
    Debug.Print "number"
Else
    Debug.Print "other"
    KeyAscii = 0
End If
End Sub

Further to my comment:

Consider a sample Userform1 with a Textbox1 and a CommandButton1

when you enter anything in the TextBox1 the change event fires - ie. typing one character fires the Change() event and passes the current value so even when you type in the negative sign your current logic fails.

What you need is to use another event like _AfterUpdate() or _Exit() with an amphasis on the second one because your can cancel the event :)

Private Sub TextBox1_Exit(ByVal Cancel As MSForms.ReturnBoolean)
    If Not IsNumeric(TextBox1.Value) Then
        MsgBox "only numbers allowed"
        Cancel = True
    End If
End Sub

You can find events here: