can we use free twilio trial account number for outgoing calls and how

On Vapi or manually in both the cases, you can make outbound calls but only on verified numbers. You need to verify the ownership of number in twilio console. If you are building something for India, Twilio doesn't provide an indian number.

Yes, but only to verified numbers, with a 10-minute call limit, and using your trial number or a verified caller ID..