IP GROUP SA - Faulty switchboard

I have installed a Cisco Switchboard system with one switchboard and 3 cordless phones. The cordless phones only worked for 2 weeks. I called IP GROUP SA in connection with the problem where they sent out an electrician to assess the situation. They noticed that the motherboard feeding to the cordless phones is damaged and need to be replaced. Apparently it is a factory issue. This happened in February. Since then I tried to get them to replace my cordless phones but with no luck. I called the service provider, I have sent them whatsapp messages. I called the rep that sold the phones to me, but with no luck. We are midway through June already, and no service. Currently I am paying for a system that I can't use, not to mention the poor quality of the calls on the one phone that I can actually use. How can I cancel this contract, since these people are clearly not keeping their end of the bargain.