Keep getting calls from this number but they never leave a message

Calls all the time. Annoyingly annoying.

This # keeps calling & texting at all hours day & night. Message says you've won walmart gift card for $100, texts say to respond to www.walmart.com to register for $1000 gift card. Texts said to text "STP" to stop them but does not work and they call & text from more than 1 number.