Welcome tapir
The first thing you'll want to do on your BV phone, after connecting the Sipura behind your cable modem and NAT router/switch behind the Sipura, is call 611 (customer service). Once on the phone with a service rep, ask for the administrator password to your Sipura box. When you have that, you can turn off provisioning on the Sipura and tweak all the settings. Provisioning can be found under the Voice tab then Provisioning tab.
QOS is a must. I have mine set to TBF (Token Bucket Filter) and the upload speed at the maximum throughput I receive (344Kbps). You can test your throughput at a place like
BroadBandReports. If you have sound quality issues, you can always knock down the QOS upload speed later on. QOS can be found under the Router tab then Wan Setup tab.
Under the Router, Lan Setup tabs you can setup how the Sipura will communicate with your NAT router/switch. By default, the IP for my Sipura is 192.168.0.1, and the next device in the chain (the NAT router/switch) will receive the IP 192.168.0.2. Just make sure your NAT router/switch is using a different range of IP's, otherwise there will be some problems. For instance, my NAT router/switch is using the 192.168.1.x range.
Under the Router, Application tabs you can set up any port forwarding. If you have port forwarding setup in your NAT router/switch, you will have to forward those ports a second time through the Sipura.
The only other thing I tweak on my Sipura is the proxy; specifically, the outbound proxy. This can be found in the Voice tab then Line 1 tab. You are welcome to leave this set to the default; I did at the beginning. It was using proxy.dca.broadvoice.com (or proxy-dca.broadvoice.com), and it proved to be quite stable for me. Sure, there were times when I tried other proxies in case the DCA one was acting up, but throughout my course of using BroadVoice (which is now over a year), it's been the most reliable...at least for me.
People will have varying results. Interestingly enough, ever since I moved a couple months ago (and therefore switched ISP's), the NYC proxy has been slightly better than DCA. I still know DCA is reliable though. Depending where you are geographically located may have something to do with it, but not always, therefore try different ones until you find one that is stable. Of course, my motto has always been, "If it ain't broke, don't fix it." So, if everything is running smoothly, don't mess with it.
A note on proxies: anything with a hyphen (-) in the name is a round robin set up by BV. Anything with all dots (.) and no hyphens is the address of that specific proxy. Before I mentioned proxy-dca.broadvoice.com, this is an internal address only recognizable by BV. It allows BV to switch proxies on the fly, so even if you have proxy-dca.broadvoice.com set in your Sipura, BV may be routing your traffic to another proxy. They do this in the event that one proxy is acting up. For example, if BV is having issues with DCA and you have proxy-dca.broadvoice.com set, they may re-route your calls automatically to their CHI proxy or something else. It's entirely up to them.
proxy.dca.broadvoice.com will use that specific proxy (note the dots and no hyphen), therefore even if DCA is having a problem, if you have proxy.dca.broadvoice.com set in the Sipura, you may experience some of those problems. If that occurs, you may either switch to a round robin address (with the hyphen) and hope BV is re-routing the traffic, or purposely specify another proxy, thus giving you total control (e.g. proxy.chi.broadvoice.com). Personally, I like the total control method.
Tips for configuring and using BV (not BYOD)? 
Linear Mode
