"Next time I am going to call the police."
Translation:La próxima vez voy a llamar a la policía.
That's just the way the idiom works, basically it is "ir a" where ir is conjugated to the appropiate person in present tense. This is how "I am going to ..." is expressed. "voy a....". You are going to "vas a". We are going to "vamos a..." etc. A very common expression, but it is just an idiom you have to learn.
There is a certain usage of "llamar la policía," but since "llamar a la policía" is over 100 times more common: NGrams, it's probably best to stick with that.
Maybe someone with deeper knowledge can clarify more about the origins of the use of "a" here. Is it an extension of the personal "a" to institutions?
But "llamar a" also seems to significantly predominate for " "municipalidad" (city hall); it's about 10:1 in Google results. For "hospital" and "oficina" "llamar a" still looks well ahead, but more in the 4:1 range. For "estadio" it's down to about 1.5:1.
Maybe one could think of those as decreasing in level of "personness," but I'm not sure if that is what is driving it (and obviously these ratios aren't certain).
On the contrary, in this case "policía" is seen as an institution, so you'll say "la policía" just as "the police".
The use of "a" on the other hand is because here "a" works as a preposition and must be used in order to establish a link between "llamar" and "la policía", even if you hear someone saying "llamar la policía" it's poorly spoken spanish.
So "hora" means time as in time of day "Es hora de..." It is time by the clock to do something. Like it is time to go to school. Es hora de ir a la escuela". And "vez" means a particular instance without reference to time of day, so "Esta vez quiero..." this time I want .... etc. If I have it right now....lol.
In English to declare present is CALL Nex time I call the police. But I am going to is to refer future is like give a warning. I am going to call . Le llamaré . In addition of this conditional situation we add Para la próxima vez. In case of.... Then. Para la próxima vez le (reference of the police) llamaré a la policía.
"para la próxima vez" seems to translate to "for the next time": http://context.reverso.net/translation/spanish-english/para+la+pr%C3%B3xima+vez