"They will have called someone else, before we get there."
Translation:De vil have ringet efter nogle andre, før vi er der.
"at ringe efter nogen" means to call someone and tell them to come.
At ringe til politiet = To call the police
At ringe efter politiet = To call the police (tell them to come)
At ringe efter en pizza = To call for a pizza
At ringe efter en ambulance = To call for an ambulance
So to put this sentence into context, imagine my friend calls me and my brother and tells us to come help him with his car. We're an hour away, so I say to my brother: "Han vil have ringet efter nogle andre før vi er der." = He will have asked someone else to come help him before we get there.