If your phone doesn't have a 3.5mm output, you can use a USB-C to 3.5mm adapter for android phones or a Lightning to 3.5mm adapter for Apple iPhones to connect the KZ Castor IEMs. Alternatively, the best-sounding option is to go for a USB Digital-to-Analog Converter (DAC) to connect your IEMs. A good DAC makes a considerable difference to the overall sound of your set up and these DACs are highly capable and outperform inexpensive adapters. If you are looking for the best presentation of your music then a dedicated USB DAC is the way to go.