This is somewhat unrelated since I'm sure your talking bout the Android app. but related all the same in terms of "Caller Id faking."
I was surfing the net and came across your question on a search engine so I wanted to let ya know about a totally free caller id faker website.
Go to
1. calleridfaker.com
2. type in your number
the number you are calling
the number you want to show up on the caller Id
Press enter and a 9 digit number will appear for you to dial. Once you do you'll listen to a 10 second ad from their sponsers then voila'! Your connected to your call and their none the wiser it's you! Until you speak that is! I do believe that that website has a voice changer function on it as well. I could be wrong but....
Oh yeah. You'll have to exit out of the little box that gives you the 9 digit number to dial every time you want to make a new call but all you do is press enter again and it will pop up with a new 9 digit number. If you try to use the same 9 digit number twice it will ask you for a PIN number.
Good luck and happy faking!!