I'm currently in an early stage of my internship at a company which offer VoIP solutions. I'm basically here to create a custom SIP-client App for iPhone. I told them however, if I were to set up the MVC pattern correctly and more efficient in terms of portability, there would be minimal code to write when porting to different platforms.
I've chose to go with MonoTouch C#.NET, for high portability and productivity (learning Objective-C is too steep for my timeframe + memory management too time consuming). To create even more portability I've been thinking of exposing a C# SIP library as webservice, so when porting to Android there's even less hooking up to different APIs. Also, MonoTouch for compile reasons does not allow usage of Dynamic Libraries.
My app would communicate to the SIP webservice and the webservice in turn to the SIP server.
SIP is very familier to HTTP, but could this solution work? As I'll be facing Realtime Transport Protocol aswell.
As far as I know, it won't work because, as you mentionned, you will face RTP. You'll probably get a lot of lag in your conversations. Also, you'll have to figure out how you are going to stream the data between the clients and the server.
However, to really know if this can be done would be to do a few prototypes to test these kind of issues.