0800-098-8906 is the technical support number for BT Yahoo but there are other ways to contact them too.
If you look at their Contact Us page, you will see other phone numbers, an option to email and also live chat.
To be honest if it was me, I would probably start with live chat as the customer support reps on those things are usually very helpful, and will advise the best method to communicate with the correct department. They may even be able to help you directly!
I am sorry to hear that your email has been hacked though, I hope there was nothing there that could lead to a problem for you. If there was, be sure to change passwords on any account they may be able to access as a result of seeing your private messages.
You might also want to consider the following tips for creating a strong passwords so that hackers will struggle to get in to your account:
- Don't use a dictionary word
- Use at least one capital letter and one number in your password
- Don't use consecutive numbers or letters that would be easy to guess (such as Abc123)
- Avoid date formats that are relevant to you (for example, using your birthday or your partner's birthday might be an easy one to guess if the hacker knows a little bit about you).