A replacement SIM is often supplied with upgrade phones incase, for whatever reason, there's a problem with your current SIM or it doesn't fit a new phone.
If your existing SIM is working and fits your new phone, there's no need to unnecessarily replace it. Just make sure you haven't inadvertently bought your new phone as a new connection with a new number, that would also explain your symptoms but mean that you have 2 separate contracts.
