Also when I try to import british number to twilio it doesnt work either. Just takes the phone and hangs up immediately

This issue is likely due to missing or invalid TwiML, an incorrect Voice URL, invalid assistant config in the webhook, or missing required fields like assistantId or phoneNumberId.

Well, I was only prompted to enter my Account's SID, secret key and the actual phone number. I have done it on Vapi's frontend when clicked import from twilio. I didnt put any voice url, so I have just default url

You do need to specify a Voice URL for your Twilio phone number when integrating with Vapi..