I assume you mean radio frequencies, and the answer is basically none. A grounded fireproof safe is basically a perfect faraday cage.

EDIT: Ok, I actually have a pedantic answer for this. If you put a microphone on a device inside the safe, you can signal it from outside by sending it vibrations, and you could encode a message in binary and thus technically send it a "digital signal". If you wanted to be a little more analog you could use Morse code :)