How can I enable Call Waiting on my iPhone?

I need help with my iPhone It at one time was able to turn on Call waiting.. right now it’s off and I can’t turn it back on

when I try.. it just says dismiss

could you please help me..

it’s important I get all my call

im disabled.. and only have IPhone as my only phone.. Please I need help!!!Thank you so much

[Re-Titled by Moderator]