The General Manager,
I want to bring to your kind notice the unsatisfactory working of our (Telephone No.) ____________. We have lodged a number of complaints but to no avail. It has not been working properly for the last one month. Either the phone becomes dead or otherwise it works one way only. So, we are facing a number of problems resulting into major losses in business.
In such circumstance, I request you to direct your staff to take a prompt action in the matter.