I'm a little confused. I understand the use of an object infix here if "simu yake" weren't in the sentence, but why is it necessary to have it here? Isn't this "he will bring it his phone"?
It's an optional way to add a bit of emphasis to the object. It's much more common when the object is a person or animal and less common when it's inanimate.
Actually there is no need for the infix here because it is clear you talk about the phone. If you continue to talk about the phone without mention it directly, then the infix is a must.
I see, not necessary, but it adds emphasis.
Still I'm generally confused : does the infix "i" indeed stand for "Simu"? Which word group does "simu" belong to? M-Mi or N/N?
N/N. Simu is singular in this sentence. The -i- used is N/N singular (-zi- would be used for the plural). In the M/Mi class, -u- is singular, while -i- is plural.
Why use this structure when the direct article is much clearer?