"I would call you" could mean different things, depending on the context.
You wouldn't use it to express doubt as such, although you could use it to indicate a condition for it happening, for example:
I would call you if I had a phone.
You could also use it with a possible reason why you would not, for example:
I would call you but I don't have a phone.
You might say "I would call you" without a condition or exception if you were answering a hypothetical question, for example:
Q. What would you do if you had a phone?
A. I would call you.
In spoken English, the inflexion you put on the word would help determine what you meant. Your tone would either express certainty or uncertainty.