Building sipXecs is when you want to change the code or when binaries for your operating systems are not available.
Building sipXecs is rather involved because it's made of of a lot of different sub projects. However a lot of time has been spent to make this as easy as possible. Historically sipXecs has only built on CentOS, RHEL and Fedora systems but that doesn't mean it should stay that way and many have started efforts to build on other systems. SUSE support is mostly there but certainly not common platform for most developers. If you want the easiest distribution to build on, the latest Fedora would be your best choice. Keep in mind, you can build CentOS binaries, both 32 and 64 bit on Fedora. Latest CentOS is probably the next best choice.
If you are just looking for source RPMS, they should all be in the respective source section in the Downloads area.