Android : Which port to use for RPC calls from Android to Netty remote server if the port 80 taken by Tomcat?

I need to implement remote RPC calls, from Android device to Netty remote server. The call originates on Android device, the server returns the reply, then the connection is closed. I plan to use something like RESTful services for these communications.

On which port should I run the service? One obvious idea would be to run on port 80 but the port 80 is taken by the Apache Tomcat server that runs on the same machine. I know that a classic Tomcat servlet can service my calls but I want to give Netty a chance.

I am not sure if it is ok to use an arbitrary high port like 8080 as probably it may be blocked by some firewall (WLAN router, GSM provider, device internal, etc).

Would it be the right approach to use repurpose for Netty RPC some other general usage port (like IMAP port 143) if there is no such service runing on the server?