Can I use a custom TTS models other than the listed ones?

Yes, by either integrating custom voices from supported providers like PlayHT or LMNT, or by using the "custom-voice" provider to connect your own self-hosted TTS API with proper streaming and authentication setup..

how?

Go through this tutorial https://docs.vapi.ai/customization/custom-voices/playht

i want to use my custom TTS not a custom voice

You can use your own custom TTS model in Vapi by hosting it as a streaming API and configuring it under the custom-voice provider with your server details in the assistant settings..