There are some 3rd party libraries and API which are used to convert TTS ( TEXT TO SPEECH ) in iphone
1) Sphinix ( Not Efficeint )
2) Open-Ear ( Do not Required Internet Connectivity offline TTS )
3) Google TTS (Requires Internet Connection)
4) Acapela (Not Free You Have to pay to get SDK)
5) Objectuve C Speech Synthesizer Speak Engine (Do Not Required Internet Connectivity One of the Good offline TTS )
The future is here with voice and natural language serve up. This is an important step in marking the service of our digital systems to us, and we have begun from the age of landlines. We have slowly evolved to touch-based palm-sized phones and computers, and now we have Apple's Siri, Amazon's Alexa, Microsoft's Cortana, and Google Assistant.
For the purpose of building voice and natural language capabilities into your own applications, one has to depend on cloud options. But if you are wondering why limit yourself only to Amazon or Apple, and create your own, you are on the right track. Anyone can build their system and enable the voice with multiple devices. It is only the difference of speech to text, followed by a query parser, a pipeline, a rules engine, and of course a pluggable architecture with mandatory open APIs.