Here is a bit of electronic security that is not mentioned often and can be beneficial for people moving numbers.
The dead drop email is as simple as creating an email account. Find a service with servers outside of your country (preferably in countries like Taiwan, Thailand, etc...)
This email address is only ever accessed via TOR. The password is known by only the two people communicating with each other.
Instead of sending an email (which gets logged by both the sender and the recipient) you create your message in draft form. PGP still applies and preferably both parties use a separate key only for the email.
An additional layer can come in the form of a digital signal, this prevents unnecessary logins. The signal can be something simple, like a prearranged status on a social media site. In addition to the mail signal both parties can have a busy signal and a distress signal.
Signals should be simple and not obvious. Like, I ate pizza today.
Food for thought.
If only prostitutes were game for this my wife wouldn't have found out.