Hello !
I ended my regualar telephone for something like 2 years ago, and I'm now using ip telephone for ordinary telephony and partly also for my and ny wife's GSM telephone's - to say it short it's now it's only allmost free ip telephony ..
I have not tried Vonage but a nuber of other vendors.
When it comes to the SME server (that I have used all the time) I will recomend to set the iptelephony "unit" (if there is a unit) on the inside (the lan side) of the SME server. Then you will have to forward the proper protocols and ports.
Different routers handles ip telephony more or less good. SME server as gateway and Linux in general does perform very good for this purpose.
When one should choose the iptelephony vendor, I think the most important criterium will be that it has to be an vendor that use standard sip technology and that has an open service where the user self is free to log into equioment, change settings, etc.
If you use such an open standard service you can first start up whith a simple standard telephony adapter, and then you can build up your telephony system later on as you like it.
For IP telephony you can choose to use only one local telephony adapter or you can choose to set op your own local telephony central using a Asterisk server, running on the SME server or one other PC.
Today I run one Asterisk server installation (Selintra rpm) at a SME server and one other Asterisk server based on Astlinux. As telephony clients I use Grandstream Handytone 486 or telephony clients running on PC. (The Grandstream is that kind of adapter that is actually containing a router and that can work as gateway, but I do not use that function. I use the inner LAN connection only for configuration.)
A ip telephony adapter at the inside of the sme gateway is an easy way to get started. Then, later you can get futher on building your own PBX using Asterisk. This will allow applying a lot more equipment, and you can use as many ip telephony vendors as you want, for inn and outgoing connections. You can also connect as many internal and external clients as your ADSL connection are able to carry (Approx 100 K for each running connection. Speed out use to be the critical parameter.)
Arne.