For most occasions, communication is usually in the form of your mobile phone. Regardless, it is important to have alternatives on extended trips and adventures into more remote locations. This can be in simple forms like an emergency whistle or with complex GPS devices like Delorme’s. This category is fully dependent on who you need to communicate with and the area in which you are communicating from.
Many of the considerations with regards to communication comedown to pre-planning and check-ins. For solo trips it is imperative to tell someone where you are going and how long you plan on being away, that way they can raise an alarm if they have not heard from you after a given time frame. Consider 127 hours, the story of an adventurer who gets trapped in a canyon for 127 hours. If he had told someone about where he was going, it is likely that he would not have had to self-amputate his arm to escape. A pre-trip communication plan can count towards your communication 4 essentials.
phone
gps system
emergency whistle
headlamp
a couple ideas of things to add to your kit.