Are the websites used for creating an AI customer service assistant, such as Synthflow, Retell.ai, and Air.ai, able to meet all the needs of creating a phone agent for small businesses, or does someone still need to code to create one that’s customized enough?

Also, websites like Retell.ai and Air.ai charge by the minute for using an AI phone agent. If someone creates their own AI agent, would it be possible to use a business’s existing phone number to make and receive calls, or would a new number be needed? It seems most people get a new number from Twilio to use for an AI customer service agent. What are all the fees for creating their own AI customer service agent?