I am looking for a software or hardware products that can solve the
following scenario:

A call comes into the office phone (PSTN) after 3 rings if no-one
answers, or if some other variable is set (eg on a web interface), the
call is routed over IP to my notebook where I can receive it with a
headset. In other words I want to be able to "man the office phone"
remotely say on weekends from my notebook.

Obviously the cheapest and most reliable products are preferred.

Are you expecting to have your notebook on broadband?

How... I'm just trying to figure this out. There are so many ways that
won't rely on the alleged quality of connection of the notebook or the
notebook's operating system. Even having your office set up for remote
forwarding to your cell phone is a cheap enough option... Unless your
PSTN (not even a PBX!) is on IP already, anything is going to start to
be expensive.

You could start with a vonage or packet8 system at office and take the
VOIP box with you and plug it into your router at wherever you plan to
have your notebook plugged in. If your notebook is going to be
anything close to dialup, the sound quality is going to be horrible.

Let me clarify by saying.

My notebook is always connected to the internet (however the IP may
change), the office has a fixed IP connection to the net over a 2Mbps
line. The notebooke uses 512/128kbps ADSL (which should provide
sufficient voice quality). Secondly I am in South Africa, thus country
specific products should be excluded.

Cell phone routing is a good option but for some reason our PBX cannot
be configured to do that due to a billing technicality. Also being in
South Africa, telecoms costs are expensive compared to the rest of the
world.

Well, see... any suggestions would start with a way to get your phone
system to forward. period. On a PBX, especially one with voicemail,
you'd have a bit of configuration options to forward on your end. With
PSTN, you're essentially a dial tone at the end of a number, so you're
reliant on your phone company to provide transfer calling feature
unless somehow there is a box that will sit and essentially three-way
call transfer inbound calls, again, supposing your phone company
supports it.

Let's start with PSTN. If you can call-forward at all, you should be
able to CF to cell phone, but that is out, so you'd have to call
forward to a service that provides voice over IP. OR, you'd have to
have your office computer have some way of answering the phone,
bouncing to your dynamic IP, and then alerting you.

Hmm... I'm looking at this product
http://www.asterisk.org/index.php?menu=home ... I'm not sure yet what
it does for your situation, but it *sounds* like it's quite the nice
thing.

Whether you use asterisk or not, I think it will save me $20,000.

Also, you'll want to set up some way to have a static IP address on
your notebook, OR go to a dynamic DNS service like www.dyndns.org to
help your office to know where your notebook is.

But surely there must be something like voice modem that can answer
the PSTN call? Then some codec compresses the call in realtime from
the voice modem driver it VOIP to my notebook say by using SIP or
something? Then I could have MSN Messenger running and pickup the
call.

Asterix is cool, but large and complicated - also would require linux
skills. I just need some small app that run windows.

The trouble I am having, is the S/N (signal to noise) ratio in the
VOIP space is so big, it makes finding something like this very hard.
There are just so many products and companies out there. Its not like
I can attribute what I'm looking for into a fancy word/product name
like "voip pstn pickup router".

:( I wish. The only thing I'd suggest is talking to your ISP about
this. http://www.afrispa.org/voiceip.htm seems to indicate some
information (I don't understand PTT and SMME) that VOIP is regulated,
but ISPs may be free to provide VOIP service.

Again, I'm pointing back to the vonage/packet8 products... while you
may not be able to start with those products, ... ooh, I just noticed
this vonage SoftPhone, provided by http://sjlabs.com.
http://sjlabs.com/sjp.html says you'll need "an account with any
Internet Telephony Service Provider for more than just direct calls."

If you talk to your ISP about providing VOIP for your office, you
*might* be able to use this program in your laptop. I'm not quite
certain how exactly, but this is pretty much what you want to do.

Thanks - I coming to believe it cant be one the way I described except
of course using maybe Asterisk.

You may want to take a look at this product from Interactive Intelligence
http://www.inin.com/products/communite/communite.asp 
It advertises find-me follow-me features.

Communite looks interesting.. I see they are the only Microsoft base
IPBX provider around. Wonder what sort of hardware it would work
with.. Dialogic card?

Not sure if you found an answer yet. I've been looking for something
similar, and like you am amazed that there does not appear to be many
products out there to do this.

I can only think its because there must be because the humble voice
modem can't deliver or there's too much money floating about just
waiting to be used to buy expensive hardware... :) With a great many
office workers having a PC connected via a LAN to high speed internet
and a phone sitting on the desk beside it, you would think somebody
would have figured out a way to do this.

Anyway, I hate people who recommend things without trying them first
themselves, but here goes... I found this website today on Google. It
looks like it might offer a solution for us. 
http://www.twilightutilities.com/