Keep using the same thread for multiple runs?

Hi, I am not sure I understand the idea of threads correctly. I am building a phone assistant and I am wondering if it would be useful to keep the same thread alive as long as possible, or if I should run a clean thread for every user query (+follow-up)?

I would keep a thread by phone number in a database. Keep in mind that run thread with large context are very expensive. I made a commercial assistant to take orders by whatsapp that keep thread by phone number and I run a new one if the last message was running 48hs ago or when the client confirm his order. Of course I use function calling to retrieve orders from my database