Have you ever gone to a concert and heard the sound engineer say "one two, one two"? If you're wondering why this is, it's because the word "two" is characterised by the silibant (hissing) sound. This allows them to test low and high frequency sounds and adjust the levels accordingly.
While this works for a concert and the audio levels for music, since the focus is on pitch variances and the overall mixing of the song, when it comes to communication, the simple "one two, one two" won't work. In this case, you should consider using Harvard Sentences.
Harvard Sentences are sentences that make use of common English phonemes in the same frequency that they tend to appear in normal sentences, thus making them representative of the language. It won't surprise you to know that they were developed at Harvard, either!
During the Second World War, scholars were working tirelessly on the intelligibility of radio communications. During this time, understanding radio messages was of the utmost importance. From this research, the representative Harvard Sentences were created (as well as the NATO Alphabet).
To test the quality of radio communications, researchers at Harvard developed a list of representative sentences. These sample phrases were later published by the Institute of Electrical and Electronics Engineers (IEEE) in their list of Recommended Practices for Speech Quality Measurements.
The published list includes 720 different Harvard Sentences, arranged into 72 lists of 10 (you can find the lists here). These sentences are still used today to test a variety of different technologies, from walkie-talkies and radios to mobile phones and Voice over IP, like Skype.
These sentences have helped develop plenty of communication technologies since the mid-1960s and continue to be used today, despite many of them sounding a bit silly!