Thesis: Help a student

Hello everyone.

I would like to write a bachelor thesis about VoIP-security.
At first I wanted to write about performance differences between RTP and SRTP, but the more I got used to it, the less interesting it became.

Now I hope for the great community here.
Would any of you have any idea what direction I could go?

Small if possible simple experiment (because there is not much time left) about VoIP security.
Asterisk, possibly FreePBX and Raspberry

Thanks =)