Translation:Next time I am going to call the police.
You could "llamar algo de cierta manera" (call something in a certain way), but not "llamar alguien" like in "calling someone on the phone".
I don't know if there could be some dialectal differences, but as a native speaker I have never used nor heard "llamar alguien" from anyone.
Edit: RAE says is an intransitive verb in this context, so it will always use "a"
Doing my own research, it seems like this has to do with llamar having two meanings. If you say, "voy a llamar la policía," someone might then ask, "¿cómo se llama la policía?" But if you say, "voy a llamar a la policía," then they will know you are using it as a transitive verb rather than the intransitive verb.
I would disagree, in British English at least you would be more likely to say next time i will call/am going to call the police, although I don't know whether in American English you might be more likely to say 'the next time'. I think Duolingo should allow for both British and American English though.