In this last section we accumulate all the "oh so obvious" issues that can make your life as an ITSP customer very unpleasant.
What is the support policy of your candidate ITSP? How long does it take to be connected with a knowledgeable person? How knowledgeable is that person? What about nights, weekends, holidays?
And also, what kind of monitoring and operation system do they have? Are they able to immediately come up with the SIP trace of the call you have a problem with? Or do they want you to provide the trace?
How are non-critical tickets serviced, like feature request or reconfiguration of features?
How does your potential ITSP handle their own infrastructure failure? What kind of High Availability architecture have they implemented? What if its datacenter got cut out from you or destroyed? Do they have a parallel datacenter? Do they depend on a single upstream provider for their connectivity? In case of an outage, you'll...