If they know about your physical device you are in a bad situation, but you would be in a worse situation if you used a phone line that they knew about. If we'r

If they know about your physical device you are in a bad situation, but you would be in a worse situation if you used a phone line that they knew about. If we're moving the goal posts then yes, there are many ways to compromise a connection, but in no situation can I see that it would be beneficial to use a landline or a regular cellphone over a smartphone with the ability to encrypt and authenticate. So why is the question how to "make a secret phone call" if a phone call is an inferior technology to begin with? If the threshold for usage was high and required specialized technical knowledge, then sure, but you can hardly say that about Signal or VPN applications.