That's something else, that's known plaintext analysis using cribs. If you know that a certain text may be present in the messages then you can try a bunch of different keys and stop searching if your decrypt contains that text to inspect for a 'hit'.
It allows for instance for the de-anonymization of network nodes based on known nodes and it allows for determining which nodes in the network are more important than others.
Like Nazis always ending their encrypted communications with 'Heil Hitler' making reversing the keys easier lol.
Although, for modern encrypted telecoms, I doubt this is as easy.