Lets just talk about a planet Mars .On an ideal day, it takes a signal from a strong radio about 3.09 minutes to get from Earth to Mars. So you would respond, “Hey Charlie, how are things?” as a communication cannot travel faster than the speed of light, the response would arrive more than 6 minutes later.
On a terrible day, the distance between the two planets can be travelled by light and radio in more than 22.5 light minutes (45 minutes for the reply). Mars is 12.72 light minutes away on average, but the animation below shows how this distance varies during the day.
On some days, Mars, Earth is behind the Sun and there is no signal at all.
Mars orbits the Sun more slowly than Earth does, which is why sometimes it is on the same side of the Sun as Earth and other times it is entirely on the opposite side.
What all of this means is that chatting to someone on the phone here on Earth won’t be like talking to someone on Mars. Voice and video messages will be packaged and sent as packets. The signal can then be decompressed and viewed or heard once it has arrived at the target planet. A response can then be sent.
Sounds like writing the letter.