your married to the job unlike electrical where you install, get paid and may never come back.
You also better have experience with whatever system you install and telecom in general to be able to diagnose and troubleshoot any number of problems that will come up. This is not something that you will find in any user manual. You also need to have spare parts on hand. One thing that you will absolutely have to understand is that a telephone system is vital to a business. Any down time means lost revenues so you better be prepared to get them up and running as quickly as possible.
Unfortunately I see many "hit-and-run" installations. They were all done by someone with no knowledge of the trade, who were usually asked by a customer to provide a system.
The data/phone wiring and telephone market is not as easy as you think. You don't just decide one day that that's what you want to do then go to the supply house and pick up a reel of cable.
-Hal
