Check your phone’s user guide or the “Get started” leaflet in the sales package to find out if your phone requires a micro-SIM or a nano-SIM. If you do not have the right SIM format you will have to contact your operator to order a replacement SIM.
Connection problem with the phone may occur e.g. if a regular SIM card has been cut to a smaller SIM format and the card is not sitting tightly in the SIM slot. You should use only proper SIM cards in your phone; do not use SIM cards that have been modified to fit the SIM slot.