Calling doctor about baby

This might be a dumb question but I’ve gotten comments like “call your child’s pediatrician” or seen it on google that says “call doctor if this” but what exactly do you say? Are you supposed to have their direct number? I’m worried my son isn’t eating enough and his discharge papers say to call doctor or pediatrician if skipping feeds or not peeing but what would i say? “i want to talk to the doctor because my child isn’t eating enough”? Is it just like a conversation or have to schedule an appointment? He has one on Friday so should I wait until then?