you need a piece of hardware to connect the landline to your internet router and convert PSTN signals to Voip and viceversa.
Such hardware's name is voip gateway and i have seen them for 70$
(like linksys spa 3102)
that piece of hardware would translate between internet Voip traffic / landline voice and would pickup calls / make calls / forward calls according to configurable rules. It would have an interface to configure the device using a local ip adress like 192.168.1.10 and there you would have dozens of options for doing everything you can imagine. Then you can use any voip / SIP software you like to place and receive calls, even on your sip client of your mobile phone.
With this i would have the power to make calls to landline in my country for free, using the data connection of my mobile phone to "talk to the router of my home" and voipGateway would place a call for me to any landline destination (which is free landline to landline) and divert the voice.
Internet router connected to landline (normal dsl service)
Voip gateway connected to both landline and the internet router
A phone company who let you make free calls from landline to landline
There are some routers which have all of this integrated, and have voip support and 2 rj45 jacks, for the connection to the DSL and Landline
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