[Symbiote]

It's the open standard for VoIP.

It's used in many places, but mostly offices. An office might have an exchange system, with features like voicemail and routing different types of call over different networks.

I have a personal account which gives cheap international calls, which I added to my android phone. I can receive calls at my SIP address, from anyone on any provider. When I make a call, I'm given the option of using the mobile network directly, or SIP.

Naturally, neither the phone networks not the big tech companies want you to use SIP. They'd rather you used normal calls, or their proprietary system.

[StavrosK]

Can you recommend one that works well? I want one that hopefully provides a Greek DID, I want to be able to make calls from my Android to landlines over my home Internet connection, as you describe, but I haven't managed to find a good (read: cheapish) provider.

[lstamour]

I use voip.ms but many others exist. Anveo, Twilio, Vonage, etc. cheapest might be a service that uses Google, but only for as long as Google offers free calls.

[allannienhuis]

+1 for voip.ms. I've been using them for years - they're inexpensive, lots of features, and service has been great.