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we need to write an application for VoIP, which would be almost 1:1 to Forfone: https://play.google.com/store/apps/details?id=com.forfone.sip&hl=en
o Incoming/Outgoing calls over SIP
o Checking for available credit
o Checking whether user's contacts are using our application or not
o Android system notifications
Please please download the Forfone application to see whether you're able to write a similar one.