Cosmopolitan Mechanical Services Inc

I purchased a tankless water heater, and the authorized installer for the company was Cosmopolitan. I did not question their competency, as the store I purchased from is very reputable.
BIG MISTAKE!!
1. About 6 months after purchasing, I received an aggressive phone call telling me I needed to get my yearly maintenance check done or else I would void my warranty. This is untrue, but the sales person continued to tell me that I didn’t know what I was talking about- until I took out the manual and read him the warranty conditions which require service at regular intervals.
2. (Much more critically)- I recently had a different gas company come to my house to give me a quote- while they were here they pointed out to me a significant number of items wrong with the install that would lessen the lifetime of my unit, not allowed by the instruction manual or the gas code in my area, and no tag referring to the installers information.
3. When I called them to get this issue fixed, they never returned my calls, although I attempted several times to be in touch with them. The store that I had purchased the tankless water heater from took full responsibility and sent their new contractor to fix- no cost to me, the issues were that serious.

I’m not sure what the situation of Cosmopolitan is, and it seems like many other people have had good experience with them, mine was just nothing close to good.