Glossary
Text-to-speech (TTS)
Text-to-speech is the automatic conversion of written text into spoken language.
In a phone assistant, TTS is the last step: the language model delivers the text of a reply, and speech synthesis turns it into audio. Modern methods generate the voice with neural networks and sound considerably more natural than earlier voices assembled from recorded sound units.
Three properties matter on a call: how quickly the first syllables can be heard, how the voice pronounces numbers, names and abbreviations, and whether stress and pauses match the content. The best test uses real sentences from the business, such as addresses, times and product names. Voices also differ by language; a voice that is convincing in English can have an accent in German.
In practice
In daily use you pick a voice and listen to it with your own sentences, not with the vendor’s sample text. Weak spots show up with phone numbers, dates, street names, technical terms and foreign product names. Much can be solved through wording: “half past two” instead of “14:30”, short sentences instead of nested ones. Also listen to the voice over the phone and not only through headphones. The telephone network carries less of the high frequencies, and a voice that sounds warm in the browser can sound muffled there.
An example
A hotel in Bavaria has an assistant take reservation enquiries from English-speaking guests. In a test call the team notices that the English voice stumbles over the name of the street and stresses the room category “Deluxe” on the first syllable. It also reads “check-in from 15:00” as “fifteen zero zero”. The team chooses a different voice and writes the time in the stored text as “from three in the afternoon”. After that the same information sounds perfectly natural.
A common misunderstanding
A voice that sounds as human as possible is not the only goal. On the phone, intelligibility matters more than richness, especially for older callers and on a weak line. A second misunderstanding: the voice alone does not make a good conversation. If the answer is too long or arrives too late, even the most pleasant voice does not help. And a natural-sounding voice does not replace the notice that an AI is speaking.
At Call0
With Call0 you choose the voice per agent. You can listen to the available voices beforehand.
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As of: 12 October 2026