Can I use Signal safely with a duplicate phone number?

I would like to use Signal without using my main phone number. So I decided to use the phone number replication app for receiving SMS such as TextNow, NextPlus. But there are concerns about security and privacy. The first is that the phone number is seen or used by someone else or by management. The second is that Signal is an application that uses a phone number to talk, so if another person leaks the phone number, you can see all the conversations.

Isn’t this related to your other thread “How to receive SMS anonymously?”?

The first is that the phone number is seen or used by someone else or by management.

Most instant messaging apps require you to prove that you own a certain phone number only when registering for the service. This is also true for Signal, WhatsApp and so on. So, there is no strong relation between a phone number and the messenger. I don’t get your question here but I assume that you are worried about another person using the same phone number in future. In this case, Signal allows you to set up a registration lock PIN. So, other people can’t easily re-register your previously-used phone number.

The second is that Signal is an application that uses a phone number to talk, so if another person leaks the phone number, you can see all the conversations.

If you just know the phone number of other people, there is no way to directly see any messages. Even if a sophisticated attacker re-registers a phone number for Signal, all previous messages aren’t readable since there is Perfect Forward Secrecy and the attacker isn’t able to retrieve decryption keys for previous messages. For future messages, any party that tries to send a message to the compromised number sees warning messages that the safety number changed.