Question: Some studies have shown that serif versus sans-serif on a computer display is not what affects readability and the practical differences between the two font styles are largely inconclusive.

I observed some designers prefer to use serif fonts only for titles or important elements and that make me ask: The font style on a CTA affect user's decisions?

Answer: 
According to [this](https://blog.kissmetrics.com/how-typography-affects-conversions/) post by Kissmetrics:

> 
> As far as research goes, there is no ironclad answer as to whether serif or sans serif is better. But, as far as general consensus goes, when it comes to reading online, sans serif typefaces are easier to read because they are simple and legible even in small sizes. This means people can spend more time focusing on the message than deciphering the typeface.
>  Sans serifs are recommended for the body of the copy and serifs for the title and subtitles.
> 
> 
> 

We can arrive a conclusion that sans serif typefaces are better for CTA moments, where people need to spend more on focusing on the message (and in this case, the message that your CTA trigger needs to give).
I would avoid using serif for the call to actions unless you have a very good reason! Sans serif is easier to read, more modern. Serif is more likely to be used in a header, where it's larger text and ultimately more legible AND suits the design values of the brand (e.g. perhaps more traditional values)