"Are there any??? -- the problem is how do you choose!! Personally, I use Skype, it's portable, works on my laptop (even my laptop), works fine under all the major operating systems, and very often runs nice free LD promotions such as the one coming up next weekend for Mother's day -- best of all, you can try it out for free for as long as you like, and even pay as you go for fancy stuff like land-line calling (inbound or outbound).

Now PBX, that's something else entirely, and the service we use at work is the oldest and most established (onebox.com) but it is also (for the same reasons) the clunkiest and most technologically backward. For example, the only way you can re-assign the on-call schedule is to login and fight with an inane HTML form page from Hell, whereas a truly enlightned PBX would know about things like webservices and 'business process integration' -- to be fair to onebox, though, I did contact a half dozen other PBX services about this sort of feature, and while Onebox is kind enough to give me a blank stare, 100% of the others did not even bother to return my email.

But what OneBox does give us is exactly what you describe, I have my extension forwarded to my (stupid) blackberry and can re-assign that as I wish, with fail-over alternates, voice mail and fax both relayed to me by email. Our company is 100% 'virtual' with members in NYC, LA, Toronto, St. Catherines and Hamilton, and where the thing works, the whole thing works very well.

And then there's Vonage, who recently, iirc, had some sort of stay put on their creditors while they tried to find a business plan, but don't quote me on that, I don't really follow the world of VoIP-wannabees that carefully. I do know, though, that the boss likes his Vonage enough to carry the box with him on business trips giving him seamless access to his phonelines from hotel rooms just about anywhere in the western world.

What I do follow is the worrisome trend to centralized services controlled by single corporations, and that's the only feature that really bugs me about Skype -- they had promised to open up their network as a public protocol, but that move has yet to materialize ... and until they do, the VoIP industry must live under a spectre of backing the wrong horse at peril of certain death.

I guess I should clarify...
I'm looking for terminations, ie, that is local numbers to dial into a sip/iax voip service which I can then connect to my existing pbx. This will let my family and cottage renters get free calling then off my pbx which is already configured for everything. They won't have laptops and my wireless service travels with me, hence no computer for skype, etc stays at the cottage.
Second, if I work from the cottage this summer, I wouldn't mind picking up some business there hence have a local dial-in number for people to reach me. The pbx can handle it, and with my wireless anywhere service, a phone there will be live with my business extension too.
Today, I've got a little trick where you call a number, it intentionally fails, gets a fast busy so you don't get billed but as soon as you hang up, my pbx calls you back (if it's from a validated number) then you simply dial the number you want...
As for onebox, it reads like the feature list of Asterisk, the do it yourself open source pbx, same as I'm running, which is some of the consulting I do to put these in for individuals and small business. Asterisk does all that and much more. If you have a decent web speed, you can host it yourself, as most of my clients do.
As for the Skype, already integrated into the pbx. Easy enough using a product called ChanSkype. I can even send instructions if you're game to set up your own asterisk box and want to integrate chanskype.
Vonage...mixed reviews, but the bottom line is it is it's own voip calling service, needing high speed at the cottage (which only exists when I'm there). If the high speed is there already, I can do it now.
On another note, what's availability and rates for DSL or radio based internet into the French Bay area between Sauble and Southampton. When I lasted checked, they wouldn't do DSL to my cottage and radio based internet ran about $600 for installation and $80 a month for seasonal service. Has this improved, any suggestions?