When I worked at Walmart I just called in sick to the local Walmart number and they connected me to the correct number.
Tell the Walmart customer service that you work at that store and then let them know that you're calling in sick and they can get you to the correct person to let them know you're calling in sick.
You can also call in sick to Walmart by calling Sedgwick at 800-492-5678 at any time.
To help you report the absence to Walmart, Sedgwick's automated phone system will also transfer you to your facility or to the associate information line, if applicable.
I've always just called my local Walmart phone number and had the customer service transfer me to a person who could take the information about my being sick and not coming into work at Walmart that day.