I’m not sure I’m 100% up to date on this but there was when I last looked no official support for Asterisk on a 64-bit platform. That’s not to say that it doesn’t work because it does but Digium don’t support that.
FreePBX does not contain bindary, it’s an uncompiled language, so the PHP installed in Apache is 64 bit.
As a courtesy Digium distributes binary RPM’s of Asterisk, however Asterisk is actually distributed as source code. If you build Asterisk on a 64bit machine you get 64 bit Asterisk capable of accessing the entire 64bit address space.
Asterisk might not be optimized for it, but the memory is there for Asterisk use.
FreePBX team creates a distribution called, “the FreePBX distro”. It seems everyone confuses the distro with FreePBX software. They are two different projects. The FreePBX distro has a 64bit version.