Skype and a few others I tested were "locked" softphones, meaning they only worked with that one service. However, by this time I had progressed to the point where I wanted a more pure VoIP experience where I could use a generic user-configurable softphone not locked to any particular VoIP service.
After all, the whole point of configurable softphones is to allow you to use one VoIP device with any, or as many different VoIP service providers you choose. Part of the challenge for me was to seek out and test the lowest cost VoIP providers who also provided the best quality, openness, and features.
After doing extensive research (this began about 8 years ago) on configurable softphones, I experimented with what few available free softphones I could find. These originally included X-Lite 3.0, SJPhone, Zoiper, and more recently the 3CXPhone.
The problem with softphones is that they are always bound to a PC. That's not a bad thing, especially when I consider I spend most of my time working on a PC. But, there are times when I just want to use a VoIP telephone not tied to the PC. This brought me to the next evolutionary step in the process of advancing my knowledge and experience with BYOD (bring your own device) VoIP devices: hardphones and IP-Phones.
After much research, I came to the conclusion that Linksys and Cisco phones were the path I wanted to pursue.
I began by purchasing a Linksys PAP2T-NA VoIP ATA (analog telephone adapter). After much experimentation and trial and error testing, I realized that this is a very good VoIP adapter. Now, with this ATA I can plug a regular telephone (corded or cordless) into it and talk just like on a regular PSTN landline phone. (Note: The PAP2T must be located behind a NAT-Router.)
My next progression was to migrate up to the Linksys-Cisco SPA2102-NA ATA. For the most part the SPA2102-NA is identical to the PAP2T except that the SPA2102 has its own built-in NAT-Router with QOS (quality of service) priority routing. No need for an external NAT-Router before the ATA like the PAP2T requires. From my experience, the SPA2102 is also an excellent ATA. (Actually, it's my favourite of the two).
Both the PAP2T-NA and thee SPA2102-NA are "unlocked" 2-Line ATA's. What this means is that I can configure two separate VoIP service providers in each box, and connect two separate analogue telephones to each box. Of course, you can't connect to just any VoIP provider. They have to be "open" providers that allow you to use any softphone, hardphone, IP-Phone, or even an IP-PBX that your choose to configure for yourself.