To build an Asterisk rpm - you need the source code and Dev tools (including openssl - bison). So why build the rpm - just compile from source. 
You will need to edit the Makefile to build for your processor type. Check the Asterisk forums for more details.
Well, there are some RH 7.3 RPMs out there, but they fail on dependencies:
[root@perserver RH73]# rpm -Uhv asterisk-1.0-0_rh73.i386.rpm
error: failed dependencies:
        libasound.so.2   is needed by asterisk-1.0-0_rh73
        libgdk-1.2.so.0   is needed by asterisk-1.0-0_rh73
        libgtk-1.2.so.0   is needed by asterisk-1.0-0_rh73
        libodbc.so.1   is needed by asterisk-1.0-0_rh73
        libpq.so.3   is needed by asterisk-1.0-0_rh73
        libtonezone.so.1   is needed by asterisk-1.0-0_rh73
To me, it looks like stuff i don't need. I am not gonna use GUI or zaptel, but some voicemail i think...
 Can i ignore these (or some) and install with --nodeps?
Per