Please read the following article:http://www.protocols.com/pbook/voipfamily.htm
With that in mind, most ways of communicating over "VOIP" use both TCP and UDP. The first one known as TCP, is used for reliable transmission of control and signaling data, while UDP is used for unreliable transmission of audio. (It's generally faster. Most "Internet radio streaming services" use UDP for audio as well.)
As TCP needs a 3-way handshake in order to exchange data, spoofing an IP-address will most likely make the 3-way handshake fail, so you won't be able to send control and signaling data if you spoof your IP-address.
UDP on the other hand, does not need a 3-way handshake and it is therefore possible to send audio from a spoofed location. The problem is that the target server will most likely not accept the data / audio as there's no "control data" being sent separately along telling the target server there's going to be an incoming call.
If you want to spoof UDP packets you can use Scapy.
I recommend you read up on TCP and UDP, even if you know it very well as you need more details about how spoofing works with these protocols. You should also study all the other "VOIP" protocols too, especially THOSE you are going to USE, as not all VOIP programs use the same protocols as far as I know.
Scapy is not
easy to use and it will require a lot of hard study on your own, including your
very own equipment too.
There's no easy way to do what you're trying to do, and even though generally it's almost not possible, remember that nothing is impossible, but what you're trying to do sounds improbable.
Please note that this information is meant for educational purposes only and that you should only test on equipment which you explicitly have permission (in writing) to test on or own legally.