There are many important areas of VoIP technology and protocols that would make fine fodder for a master’s thesis in telecommunications (an engineering degree, I would imagine).
According to what I read, the following areas are of particular interest and concern in the field right now:
1. Noise and echo cancellation and data compression technologies: In a nutshell, all of these data manipulation techniques seek to remove noise from voice communications, and to reduce the amount of bandwidth that must be utilized to transmit them from sender to receiver. For those with a strong mathematical bent, this is a great area for research and development (and possibly even technology or algorithm patents).
2. Security topics include detection and blockage of SPAM over Internet Telephony (aka SPIT), improvements to mutual authentication between call initiator and call recipient, detection and enforcement of media connectivity (SIP doesn’t check for connectivity, yet it’s essential for connectivity to exist and be maintained for VoIP communications to succeed and proceed).
3. Detection and avoidance of unsolicited and unwanted calls is not only likely to be a great boon to VoIP users everywhere, but also likely to result in technologies of great commercial interest.
Hopefully, there’s something in there to attract your interest.
Good luck with your research and writing. It sounds very interesting.