Struggling to sort this out for a year

We have installed a PABX System at our offices but ever since then, our lines have been too soft that we struggle to hear the other person on the other end. we have logged in a complaint a numerous times, and different technicians came over, but it could only work properly for a short while then going to being soft or make a flickering noise.<br> <br> We have been seeking assistance on changing the unit many times but haven't got any. This is affecting the company. Please help.<br>