Install PABX telephone system

Hi, I have an old PABX telephone system that is playing up. I've bought a second hand system that I would like installed in its place. The business its getting installed in has 3 or 4 pstn lines coming in that are used for calls with line hunt, fax, internet and chubb security. The lines are currently playing up so it might be the pabx or some of the joins which you will need to problem solve as well. Telstra say the problem is not external to the building. The system has one main reception phone and will have 6 extension phones. All cabling is in place to the extensions.