I am new to sip trunking, I am looking at the documentation and couldn't find much information on how sip trunking works and how to configure that via vapi. Does every telephony provider gives us SIP trunk gateways? Or do we need to create our SIP gateway and route all the calls through our own custom gateway to any of the telephony providers phone numbers? I have gone through the below documentation. But, no luck https://vapi.notion.site/SIP-104008c8393f80feaeb7de8ca19565ed