Opsec Phone

Here's an article on making secret phone calls with cell phones.

His step-by-step instructions for making a clandestine phone call are as follows:

1. Analyze your daily movements, paying special attention to anchor points (basis of operation like home or work) and dormant periods in schedules (8-12 p.m. or when cell phones aren't changing locations);
2. Leave your daily cell phone behind during dormant periods and purchase a prepaid no-contract cell phone ("burner phone");
3. After storing burner phone in a Faraday bag, activate it using a clean computer connected to a public Wi-Fi network;
4. Encrypt the cell phone number using a onetime pad (OTP) system and rename an image file with the encrypted code. Using Tor to hide your web traffic, post the image to an agreed upon anonymous Twitter account, which signals a communications request to your partner;
5. Leave cell phone behind, avoid anchor points, and receive phone call from partner on burner phone at 9:30 p.m.­ -- or another pre-arranged "dormant" time­ -- on the following day;
6. Wipe down and destroy handset.

Note that it actually makes sense to use a one-time pad in this instance. The message is a ten-digit number, and a one-time pad is easier, faster, and cleaner than using any computer encryption program.