What do you do with a company who did a bad install who will not come out to inspect the job or return your calls?
I'm almost certainly in a different country form you so my answer may not apply - but you may have some equivalent options

I would write to them 


stating the problem clearly, 
listing the attempts made to get a response from them (with dates)
and saying that time is of the essence (legal phrase in my jurisdiction)
saying that if they don't contact me within n days to resolve the problem
I see no option other than to get legal advice on what steps to take.
(n needs to be a reasonable amount, perhaps 14 days rather than 2)


I would send this by registered-post or recorded-delivery. It makes more impact if someone has to sign for it.

Keep a copy of what you have sent. Copies of written communication may carry greater weight later than your transcript of what you think you said in a phone call.

In my locale there is a trading-standards body to whom consumers can complain about
shoddy work or goods. I'd check if they could help.

In my locale there is a small-claims court which can be used for low-cost proceedings
there is a limit on the size of claims, hearings are heard in front of a judge and
court-fees are low. I would check whether this service is available and what limits apply

Getting a lawyer to write a short letter to a company may help, even if the letter is
relatively innocuous it can help persuade them that you are unlikely to go away quietly.