For a typical ELT,
take a look at this Honeywell brochure and look at the features for a fixed installation (two of the models). Other manufacturers offer similar capability, as a variety of standards/specs are out there for compliant equipment. (No, I don't work for Honeywell. Thales and Artex make them as well). You could also read back a few pages, within five of this one, and find a variety of posts regarding ELT's.
This is one such post . Within a page either side of it is some more info. You could also take a gander at the ELT threads in Tech Log.