Estoy llamandola

[Tyrn]

Hi,

I just can't get my &#*^ together with regard to when I am allowed/required to use
a leading/trailing pronoun. I can't even google the subject effectively because I don't know the keywords.

Context Reverso definitely favors la estoy llamando

[Rusty]

It can go prior to the progressive/continuous form or it can be suffixed to the gerundio (the second half of the form).

When suffixed, an accent mark must appear over the vowel in the third syllable from the end (to maintain the original stress of the gerundio); e.g., estoy llamándola.

[Tyrn]

Is it all there is to it? Duolingo never rejects a sentence just because of the missing accent, but it's obviously opinionated about prefixes/suffixes. Arbitrarily, as it seems to me More than that, I vaguely remember something from a forgotten textbook about when and where one has to use this or that, and when it's a matter of taste.

[Rusty]

The pronoun can go in either location, when a verbal is in use. There's nothing more to it, other than one's preference at the time the sentence is formed.

Duolingo frustrates a lot of people. It seems they believe there's only one right answer, but natural language is not so rigid as that.
Using proper accent marks may be lost on the current generation, but it is still regarded as necessary by Spanish academics.