I have found servers items that will cause this issue.
1. first does is always happen or does is seem random.
2. were windows stores the paging file is there more then .1 gig available
3. check available ram mem over time
4. does your client use a TV carrier such as bright house.
I have found most of the time, the issue has occurred when connecting to the IPO was via analog lines. The carrier would bring a T1 to the prem then convert to analog lines.
Items one, two and three are a server issue. Item 4 is harder because you have control over the hardware. I would asked the client to setup a log book and every time this issue occurred, document time of day, phone number and asked their customer who they used as a carrier.
I always requested the carrier to check the line voltage prior to the connection to the IPO because getting the carrier to accept the blame is very difficult and this is a go start. Also you have the ability to set the analogue impedance match. go to line\analogue options\start stop then save settings.