Generally, things are pretty simple if you're looking for a hosted service. Many of the top VoIP providers handle all the heavy lifting offsite, delivering calls to your phones and software clients without much hassle, especially if you use phones that are plug-and-play certified for the service in question. The majority require no additional on-site hardware aside from those phones; at most, you might need to find a space for a small box of hardware somewhere on-site.
In contrast, maintaining a self-hosted, on-site VoIP system requires a bit more work. You need an IP-based private branch exchange—a VoIP-friendly version of the PBX phone systems that many offices use—to route your calls to the appropriate phones on your network, as well as a device called a PSTN gateway. The PSTN gateway sits between the IP-PBX software and the analog signals of the public switched telephone network, converting calls to and from digital signals as necessary.
No matter which option you choose, typically you can handle the basic settings for your phone lines or extensions over the phone, while tweaking more advanced options requires diving into your provider's online account interface.