"I do not have time to call him."
Translation:Jag har inte tid att ringa honom.
Basically speaking, för att is "in order to" and att is "to" as an infinitive marker.
The problem with the latter is that modals never trigger the att, and Swedish has a rich modal system. As for all verbs that are not modal, the att is either optional or mandatory, and you have to learn which is which by heart. I usually recommend always using the att with non-modals and learn the idiomatic exceptions as you go.