The phone has to be unlocked. Since it's a Fido phone it probably isn't. If the phone is off contract sometimes Fido will unlock it for you. (Call them and ask.)
You may also be able to get it unlocked at an independent cellphone store. But I'm not familiar with SE phones so I don't know how feasible or expensive that would be.
Best thing to do if you have a SO (or other non-Fido) SIM is to try it out in the phone before you buy it.