If you are a VAR or telecom agent about to pick out a strategic partner to be the VoIP Provider that you work with, here are some things to ask the execs.
Are you a CLEC? It isn't a requirement to be a CLEC to deliver VoIP. However, if not, E-911 and porting will be done by a third-party. Find out who. This brings up the other similar questions:
Can they port numbers? Most businesses don't want to lose their phone numbers or toll-free numbers, so ask if (and where) they can port numbers. Many VoIP Providers back-end into Level3 who does the porting and hands them DID's (phone numbers). But you need to know the territory that you can sell in. Plus who is the RESPORG for transferring toll-free numbers? Maybe how is E-911 handled?
What is the Disaster Recovery/Business Continuity plan?
How do they handle Fax over IP? T.38 works for the two-page occassional fax, but if the business does a lot of faxes (lawyers, mortgages, etc.), TDM is the name of the game. T.38 requires a compatible fax machine, IAD, and network.
How is NAT Transversal handled? If some employees work from home behind a Duopoly broadband connection, how will the IP Phone connect to the LAN?
Not every IP-capable PBX can connect to every VoIP Provider. Some do Inter-operability testing. Some don't. A duct tape solution is to do PRI conversion of the SIP Trunk at the customer premise, but that will be messy. PRI is a standard with just two configurations in any class 4 or class 5 switch. However, SIP is a collection of about 30 RFC's - or think of it is 30 compromises that were never standardized for the industry. So you have to ask about inter-op between the SIP switch or SBC and the hardware before the order.
In a similar thread, you might want to ask how some special features that you are used to are implemented (if they are). For example, overhead paging, call park, message light, and other key system features. If they are important to the client (due to what I call Blinky Light Syndrome), you want to make certain that you explain the implementation to the business BEFORE you order.
Finally, how do you maintain Quality of Service for the VoIP packets? Is this On-Net via Private Line or MPLS? Or does this service ride the public Internet?
One for the Geek Squad:
What codec to they use? Important only so that you can talk about it with the geeky prospects, but also so you have a handle on the upstream bandwidth needed. FYI: G.711 (64 kbps); G.729 (varies); and G.722 (HD).
Obviously, there are questions about the VoIP Providers commitment to the Channel; how quotes and orders are processed; commissions; and the like, but those are the same questions you would ask a cableco, cellco or CLEC.