A long time ago someone asked the same thing, with no responses. I would guess that for business customers, yes, but residential service, no. https://www.3cx.com/blog/news/uk-voip-provider-bt/
You might just ask them directly. If they won't provide you with things such as a password, user name, server address, then you know that they want you to use their hardware, but they should be able to tell you that. If they insist that you use their ATA, then you could Gateway it into 3CX. Not the most economical, but it would work.