Expense of gateways
One issue I've seen is that moving a small office from BRI (ISDN-2) to SIP is that the existing PBX power costs are going to go way up. We threw together a gateway out of an old cisco 3600 router and what used to be very expensive line cards. Its power use for the year would have exceeded the ISDN line cost. A new all VOIP phone system would also use a massive amount of power compared to some older phone systems. Our large office still hasn't used any of the cool VoIP features of its over priced phone system even though there are claims the old system just couldn't do it.
The power per phones is can grow quite a bit. At 2.5W max for the old 6 line analog phones with the big displays, they use 1/4 of the power of some of the fancy VoIP phones we looked at. Since these are on 24x7, their power costs should be considered since it may include new costs costs such as PoE switch, switch power, increased PBX power and increased phone power consumption, UPS and cooling for the switch/PBX.
I would like to get one of those patton gateways (mentioned above) but simple google foo doesn't even come up with vague lists and have the "call us to talk" mentality so they will fall off the potential supplier list fairly quickly.
Another issue is that SIP systems are often excluded by national telephone provider rules so the end customer is required to pay for all fraud even if the SIP provider doesn't even do simple protect against things like credentials from the wrong IP address or "Fake False Answer Supervision."