The Apple-inspired deletion of mini-jack output sockets from many smartphones has had unexpected consequences. Headphone manufacturers have responded by selling models terminated with USB-C plugs, but there are two types, one of which is not compatible with all smartphones.
Active The first, 'active' type uses the USB interface as you would expect, to convey the audio signal digitally. For this the headphone must contain a DAC (digital-to-analogue converter) and amplifier, which are normally powered via the USB interface. This adds complexity and cost, of course.
Passive The second, 'passive' type is a conventional headphone which relies on the smartphone's USB-C socket also providing an analogue audio signal. Properly known as Audio Accessory Mode, this is not supported by all smartphones, which is where the compatibility issues normally arise.
Matters are not helped by headphone manufacturers not always making crystal clear whether USB-C terminated models are active or passive, and many smartphone users have no idea whether their smartphone offers Audio Accessory Mode or not.
The simplest way to avoid compatibility issues is, of course, to try before you buy. If this is not possible then you can usually glean whether a USB-C headphone is active or passive by checking its detailed specification. If an impedance and/or sensitivity is specified, then it is probably a passive type. I've searched online for a listing of smartphones which do and don't incorporate Audio Accessory Mode but thus far have been unable to find one. If you know of such a resource, please use the form on the Contact page to let me know and I'll publish the link here.