you have checked that it is clear
The only way you can tell for sure that your phone line is clear and that you are not still connected to scammers, or anyone else, is to call someone you already Know And Trust. Only when you actually speak to someone you already Know And Trust, like a relative or good friend, can you be sure that your phone line is free and it is safe to use it to call your bank etc. to tell them about the scam call.
When you do, make sure you ONLY use a trusted phone number for them, i.e., the one on the back of your credit/debit card, or on a statement you already have. If you need to check with or tell your friend or relative about the scam call, be very quick, as you urgently need to call your bank/building society (or whoever the scammers said they were calling from). It is a good idea to tell someone you trust, but you do need to call the
bank/company first. However, if you're not at all sure about doing that, do speak to a trusted friend and get help before calling anyone or doing anything else!