Personal electronics are the easy category at customs. A laptop, a phone or two, a camera, and accessories clearly for your own use travel duty-free into virtually every country under personal effects rules, with no registration or declaration needed. Officers care about quantity and newness, not gadgets as such: three sealed phones in retail boxes read as goods for resale, while your obviously used devices read as luggage.
A few countries do have registration schemes worth knowing about, mainly for expensive gear you plan to take back out again, so you can prove on return home that you did not buy it abroad; your own country's customs may offer this too. Some destinations also require local registration of phone identifiers for long stays before a foreign handset keeps working on local networks, a telecom rule rather than a customs one.
If you carry unusual amounts of equipment, keep receipts, and when in doubt use the declaration channel. For ordinary travelers with ordinary devices, there is nothing to do.