Not to jump in at the end, but how much time do you think you have spent on this project versus how many more projects you could have gotten that you KNOW and would/could do at a very high level of expertise? You probably could have done two more jobs that you KNOW how to do by now, which from a business perspective would have been many times more profitable.
I have to agree with the others. If this was something you were an expert in, then you would have been fine. Since you weren't/aren't, then you should have had replacement parts to get the system back up and running at a moments notice, (even though phone systems tend to be rock solid as you have seen). If you didn't have the means to get replacement or backup parts, then you should have at least had another plan of action for the customers sake rather than posting on the internet looking for help as their system was "dead in the water".
Just my two cents...
