If you sell someone a system and it doesn't work or doesn't work the way it was sold, then you are to blame. If the system has a limitation that you thoroughly explained to the customer, then tell them "Hey, that's Life". If the problem is outside of your control then what can you do?

I get phonecalls about my phone installs every time it rains. Verizon's copper is cutting in and out and therefore it must be my box. Similar situation as your 512K pipe.

The glaring issue with whoever installed those phones are the 512K pipe. They probably dont have a QoS router installed to make sure the phones get priority either.

As far as the phones being inline it would depend on the phone. Some phones will actively act as a QoS limiter and cut back the computer traffic if it gets to high. This is the case with Polycom. The other side-issue is that now you have 2 lines ran to your phone and it makes moving it a PITA. Dont get me wrong, it's still done cheaply tho. It's one of those things where you go "Yeah, it'll work, but that's not what I recommend and I wont guarantee it" and explain why in detail. If they still insist on it then you either drop the job or let them get what they want.