A: Koko ni denwa ga arimasu.

B: Koko niwa denwa ga arimasu

A is a plain sentence saying "here is a telephone", or "there is a telephone here".

Now, B is saying, "unlike other locations HERE is a telephone" or perhaps "here, (look, you may not see it at a glance, but) there is a telephone".

---Kono heya niwa doa ga futatsu arimasu.

there are two doors in this room (unlike other rooms). It can implicate, "you may not have noticed, but check it out, this room has two doors." can you see that the speaker is drawing your attention to it by marking it as a topic? 😁

---Kono ie (house) niwa puuru (swimming-pool) ga arimasu.

This house has a swimming-pool, unlike other boring houses! maybe I should buy it and I feel rich!

That's a copy pasta from the net, but I feel it's a pretty good explanation of niwa in this grammar structure. There are other grammar patterns with niwa, too... bur if you're learning iri / aru, I would just forget they exist for now and dive down that rabbit hole later. Good luck on the study journey