I doubt an app could detect a call that was being forwarded. That's stuff that would have to be looked at as a carrier, incoming/outgoing calls to the forwarding number.
Before I've heard back from my mate regarding cell towers. I was thinking this is going to have to be split in to two different methods, one for calls and one for sms's.
For SMS's there is a lot less security on number spoofing, if anyone wants proof send me your number and I can text you from any number (eg: 911) or 11-letter word (eg:TheIronDen). It's done through an SMS gateway, if you sign up to commercial ones you have to verify your mobile number and it will only allow you to send from that however there are ways around it. SMS gateways are what big corporations use to send mass messages from. At a core level, you could essentially send anonymous text messages by the use of an SMS gateway. I've got a script written up in MSL to run a bot on any irc network which allows this to happen. It wouldn't be very hard to write this to an app, have the app relay the details to an offshore server which executed the sending of an anonymous text message. This would be traceable to offshore but doubt it would be stoppable or traceable.
As for calls, I think it would be easier to spoof them to another phone as opposed to truly anonymous (this would still allow calls to not be traced back to you). I assume the subscriber ID is checked at a network level when calls are made to ensure the user has credit, etc. so when making calls you would need to spoof your IMSI to mimic that of a legitimate IMSI. And funnily enough, that user should actually get charged for the call too.
I know with a custom iPhone firmware, developed for iPhone forensics, it is possible to spoof/change some of the outgoing details to make this possible. I need to know exactly what details the cell tower requires first.
I've been doing some wireshark experimenting with an android phone lately but that only captures Internet data not cell data. I'll see if there is a wireshark equivalent to work on calls as opposed to 802.11