Your two sentences tell the same story, but from two different points of view.
The first is a present-viewpoint sentence; the second, a past-viewpoint sentence.
At this very moment I have still not rung because I have had so much
work to do during this most recent time period. (The situation
described here still exists at the moment of speaking.)
Some time ago I did not ring because I had so much work to do during
that time period. (The situation described here no longer exists.
It is a situation which existed in the past.)
If we construe "some time ago" in the second sentence as "some
very short
time ago", it approaches the meaning of the first sentence more
closely. Exactly how long ago is not something which is specified
by the past tense itself.
CJ