I have a SIP account (SIP is a protocol for voice-over-internet-protocol – VoIP) at OVH (a web hosting provider). I could choose, when creating the line, between various « national » numbers (France, Belgium, etc). I choose a french number (even though I live in Belgium), since nowadays most internet providers in France provide, through their « box », free and unlimited calls to french landlines, which therefore allows my friends and family to reach me at zero costs.
The line is activated through a Cisco SPA-112 phone adapter, a two port « ATA » (Analog Telephone Adapter) that allows the use of basic analog phones. Currently a simple DECT phone is connected to this adapter.
You can also activate the line through a software (softphone), such as Twinkle for KDE users.
My final setup involve an Android phone, the SIP softphone « CSIPsimple » (downloaded from F-Droid), which allows me to place calls through this account whenever I have a data access. Of course, as soon as you register you line with the softphone, the incoming calls are redirect to it.