2. Requirements

To spoof a call, we need to implement VoIP first.
Basically we need 3 major things: PBX, SIP Trunk and a Softphone.

1. Any PBX will work, generally asterisk is used.

2. The issue comes when finding a SIP Provider. 
Earlier goTrunk was used but now they have changed their server to not obey the "CALLERID=" command from Asterisk, disabling caller ID spoofing.
There are many paid ones that work like VoIP.ms, Flowroute etc. But we need to find a free SIP Trunk that does not report DID verified status so 
the reciever's SIP trunk just has no clue it's a fake number and accepts the call.
After days of searching, luckily I found a free SIP provider. I am providing the details with the hope that it won't be exploited and people will use it for knowledge purposes
only. It is called "ringplan".

3. Any softphone like xlite, zoiper, linphone, microsip can be used.