If you're asking how to say "I was not at home when you tried to reach me (e.g. at my parents' home) but was on the campus of the college I attend"...
In American English in college would be incorrect, for the reasons I gave in my comment. I was in college can be paraphrased I was a college student. It refers to a time period of one's life devoted to studies at university level, not to one's physical presence on a campus. Only in contexts where that temporal meaning makes sense would it be idiomatic: The famous poet died when I was in college.
However, "at college" alone is not fully idiomatic in AmE either. We would probably say:
I was away at college when you phoned [my parents' home].
Or, if the call did not go to your parents' home but to you, you could say:
I was on campus when you phoned. Reception there is very poor, and I did not get the call.
That statement is made with the speaker assuming the listener knows he is a college student (or teacher), or can readily deduce that fact. You could also say:
When you phoned, I was on the campus of the college I attend [or where I teach].
if speaking to a person who knows nothing about you. You would not say that to a buddy from high school with whom you keep in touch, who knows you're in college, and the name of the school.
P.S. So the upshot is: these are collocations and you cannot presume an abstract rule about the preposition in isolation will lead you to an idiomatic choice.