I have been working with Packet8 using their SIP service for a month or two. We had a few issues which Packet8 has been pretty active in addressing.
The Packet8 service relies upon an adapter device. The device is a small black box that sits anywhere on your network. In the initial setup the device gets a dynamic IP address. Since the Response Point configuration for the SIP service uses the device IP address for the Proxy Server Address field - it looks to me like you are going to want to make this device a static IP device which you are able to do through its Web interface.
Initially I experienced a few instances of dropped calls. Packet8 was pretty quick to get me a firmware update for their device. Knock on wood - I have not had any more dropped calls since the last firmware update.
A feature that I have seen with several SIP service providers is what I will call a "failover" routing. If your network is down or off the internet for whatever reason - what do you want to happen to your incoming telephone calls? Without a failover routing - they will not go through - your customers will get an error message or dead air. With a failover routing - the system can be provisioned ahead of time to send your telephone calls to an alternate telephone number - hopefully you will specify a phone number that is not subject to the same outages as your network or telephone system - like the owner or a manager's cellular telephone. I think my buddy Ed Carnes experienced such an outage just after connecting with Packet8. His ISP had an outage and this was before Packet8 had implemented their failover routing.
I have requested that Packet8 make the failover routing an item that is accessible to the customer via their website interface.
Another feature of many SIP providers is what I will call Caller ID spoofing. Now, by spoofing I don't mean anything improper or illegal. In my case, I maintain my main business telephone number as a land line with my telephone company. This gets me a listing in the real telephone book. My Response Point system therefore has both analog and SIP lines connected to it. My outgoing calls go via the SIP service. HOWEVER, I do not want to confuse my customers by showing them some unfamiliar Caller ID number - I want them to be shown my published business phone number. Without doing something extra - the system will not work this way. Some SIP services accept the Caller ID field that the Response Point Administrator configuration provides. Some other services do not honor the Response Point Caller ID field, but have a field in their website customer management screens where a Caller ID record can be entered. Packet8 has advised me that they are working on a way that the Caller ID number can be specified.
The pricing picture varies significantly among the SIP providers. Packet8 sells their service, for the most part, based on pre-purchased blocks of outgoing call minutes. These minutes count for both local and long distance calls but only for outgoing - the incoming are pretty much unlimited. One SIP Trunk can, in their case, can accommodate multiple telephone calls - up to twelve (12). So for many small businesses, there should not be a situation where all 'lines' are in use and a caller gets a busy signal.
An issue that is a somewhat up in the air currently is the exact pricing plan for the blocks of minutes. A couple VAR's have been working with Packet8 trying to get some clarification and simplification on the pricing.