Complex sentence: Some aftermarket mobile headsets come in a standard 2.5 mm plug, so users have to purchase an additional adapter for their mobile phones; a USB headset for a computer also cannot be directly plugged into a phone's or portable media player's micro-USB slot.
Split into simpler sentences: Some aftermarket mobile headsets come with a standard 2.5 mm plug different from the phone's audio connector, so users have to purchase an adapter. A USB headset for a computer also cannot be directly plugged into a phone's or portable media player's micro-USB slot.