skype-java-api | Skype API for Java , based on Skype4Java library | REST library

If I have well understood, the use case is:

• A wants to call B through an application running in a mobile device
• B has a phone land or mobile line, but not a VoIP one to receive the call.
• Bridge between internet and phone lines is to be done at home (A's home) without specific subscription costs, that is to say, without the services of a VoIP provider (I should like here to suggest rethinking the use of a well stablished solution as costs to call phone lines from IP can be really cheap).

Well, there is a lot of solutions for this scenario. I am going to speak about one of them that I consider interesting because it opens the way to a lot of additional communication services.

First, the softphone. To make and receive calls, A will need an application in his or her device. Consider a softphone as Zoiper or Jitsi Meet.

Then, the gateway between VoIp and phone lines. Asterisk can do the work as a SIP server. It is a lightweight linux software with a lot of features. It can switch VoIP lines with land phone lines via FXS - FXO cards (if the phone lines are analogue ones), ISDN cards, VoIP interfaces, bluetooth using mobile devices, etc.

Last, but not least, the connection. Ok, you do not want to expose your gateway to the dangers of all those wicked people of internet, eager to stole your phone line minutes. Connection between mobile and server could be done using a VPN (e.g. OpenVPN), or through a web app (SIP on top of WebRTC).

Once you have the asterisk working at home, you could use it as an answering machine sending email messages with the received audio, as (if your local regulations allow it) a recorder, as an IVR or as a part of a security system, calling sequencially phone numbers in case of emergency.