There are 2 ways to do this. you can buy a used so called thin clinet pc. These are typically amd 1 ghz or simular type small pc's. You can install eleastix or freepbx (asterisk based on these) and hook them up to your own computer.
I just rented a vps that had asterisk/elastix installed. Minimum is 512 mb for it to run well and som good cpu speed. Depending on your location this should cost between 8 and 20 euro per month.
Setup is not as easy as on pbxes, you need to setup trunk and supply some config lines.
Outbound / inbound routes work simular but take some testing to work.
My company only works on voip and I have 10 inbound trunks, did numbers from different countries. Took me 1.5 days to setup but if works fine now. There is no 3 callback numer limit.
I also use voicetrading for callback to my uk cellphone but you might want to checkout
SmartVoip | The smart way to save on your calls! they only charge 5.5 cents (without vat) to english mobile number. You can transfer money for your voincetrading account to smartvoip.
Smartvoip does not allow forwarding of caller id so you will not see who is calling you. also smartvoip like other betamax clones (except voicetrading) are now blocked on most pbxes servers.