Open the dialer app > menu (vertical ellipsis top right) > Settings > Calling accounts > tap the SIM settings at the top > tap Select UIM/SIM Card = to find the settings regarding Voicemail, Call forwarding etc.
It's a separate setting for each SIM card.
Please note that call waiting, call forward etc. are operator features and your options vary per mobile operator/carrier. Additionally, your mobile operator may provide a self-service website to control these settings and more in a browser.
Does this help?