I'm going to tell my mum to stop giving out her number
Why would you tell her that?
My ex-boyfriend just keep on calling her to chat.
It's fine to give people constructive advice. Otherwise they may keep doing things that are bad.
I don't want my boyfriend to talk to my mom anymore either.
You should talk to him about that. It is weird for your boyfriend to call your mom for casual conversations so tell him how that makes you feel.