If the rain had stopped I would call Peter

[5jj]

By definition, "If the rain stopped/were to stop..." implies that it's unlikely for the rain to stop..

It doesn't.

 

[EngLearner]

The following is written on this website:

Conditionals: if

Conditionals: if - English Grammar Today - a reference to written and spoken English grammar and usage - Cambridge Dictionary

dictionary.cambridge.org

There are different types of conditions. Some are possible or likely, others are unlikely, and others are impossible:
...
If the weather improved, we could go for a walk. (It is not likely that the weather will improve.)
...

 

[5jj]

1. "If the rain stops."
2. "If the rain stopped/were to stop..."

Absent more context, the stopping of the rain is seen as less unlikely in #1 that in #2.
Absent more context, we have no evidence that the stopping of the rain is unlikely in #2.