I think the best practise is Skype, which, I guess, you're already using. But if you'ld like more functionality, more additional features you can set up an IP PBX platform.
To do this you should:
1) disscuss what features do you need;
2) refer to your telephone company specifying the possibility of IP prolongation;
3) configure and setup of the hardware (IP phones and PBX).
By deploying IP PBX you'll get short numbers for inside company calls, possibility to unite separate offices into one system, call redirection, call forwarding, fax, fax to email (as attached file). Besides that you'll be able to monitor all the telephony processes through the WEB site, available anytime and anywhere from the Internet.
Here is small amount of additional services you can get:
-Automatic configuration and reconfiguration of the phones
-Login to your phone number from anywhere from the internet
-Saving of telephone numbers of the office, if changing location of the office
-Dynamic enlargement of the telephone lines or numbers if necessary
If intrested in such a solution - contact me on skype for further disscussion ANDREW.IXC