Here is the sentence.
Where were you yesterday? - I was at home the whole day. - How strange. I (ring) you up at two o'clock, but nobody answered.
The Past Simple would be: I rang you up at two o'clock, but nobody answered.
The Past Continuous would be: I was ringing you up at two o'clock, but nobody answered.
I am wondering why the authors of the grammar book where I took the sentence from tell me that I have to use the Past Simple. In my opinion the Past Continuous is more appropriate, for we have the exact moment of the action in the past.