We have been using our VOIP only solution now for about ten months and over time the experience has got better as we have learned a thing or two.
Our provider is Voiptalk to begin with they were a little bit flakey but they seem to more stable now. I think their servers are located in a docklands datacenter and there has been four brief outages over the last ten months which is no problem for us.
Our Asterisk server has been rebuilt a few times but now seems to be stable, it is running on a Dell 1Ghz PIII with 512MB of ram, we used fedora core 3 for the os and the standard free version of asterisk.
We have eight phones in the office and are using features like voicemail and out of office redirection to mobiles.
We opted for Polycom IP300 handsets @ £100 each these represent good value over extensive functionality.
For a long time our biggest problem was call quality / calls dropping out, this was remedied by turning on traffic shaping on our m0n0wall router which is our gateway to our 2mbps 5:1 SDSL line supplied by Bulldog.
The Bulldog SDSL line here in Tunbridge Wells has been rock solid, I wish I could say the same about there customer service. BD have blocked port 25 so we relay mail through their server, recently for whatever reason their SMTP server has been causing our outbound email to be delayed which is very confusing / annoying. I have contacted them on several occasions but they have offered no way of resolving this.
Adam commented below that Bulldog don't port block and he is right. We misdiagnosed that issue, however, when we started relaying mail ourselves we got a lot of NDR's with DNS errors, the errors stated that MX records for the domain could not be found. I can't be sure this is a Bulldog DNS problem but I have my suspicions. Of one thing we are certain, their outbound relay servers do delay messages occasionaly, the delay can be up to a few hours long. Our experience of their customer service is poor.