@jamelia
Intermittent failures can be caused by faulty electrical connections.
Test your SIM card in another phone for a while, if you can. Test another SIM card in your phone for a while, if you can. This can help identify which hardware is causing fault.
You can clean electrical contacts with a pink eraser or clean isopropyl and lint-free q-tips, toothbrush, etc - there's tons of info all over google about the specifics. There's often near-invisible oils/residue which interrupts proper electrical contact. There's sometimes near-invisible corrosion caused by moisture or galvanic fretting.
You can also clean the electrical contacts inside the phone's SIM slot, assuming they're physically accessible. Remove any dirt or debris. Straighten any bent or misaligned pins. The metal contacts can eventually deform and lose the necessary mechanical springiness. The solder points can be cracked from excessive force or impact. The gold plating can be worn away enough to oxidize the underlying substrate or to just prevent a solid friction-fitted interface.
