VDSL line drops

I signed up for a VDSL line with a download speed of 40mbps, a year ago. This speed could not be maintained and we agreed to a 30mbps line. However, this line has not been stable and recently our speeds have been dropping to as low as 4mbps or there is no connection at all. At present our phone line is crackly and a technician was sent out on Sunday. The problem persists. I have tried calling the centre twice today and both times I have been cut off. Please can the line be stabilized. If the connection is below 30mbps, it would stand to reason that our billing should be adjusted to a 20mbps line. We are currently paying for a very unstable service and at times a service that is non existent.