I can't trust any company that has to read my contact list PERIOD! It's not something anyone should be having to share ever.
On the contrary, if you answer the same to WhatsApp, it plain refuses to work. But it actually created an account on their servers, and from that on you appear on your contacts who do use WhatsApp as another user of WhatsApp, which invites them to write to you there although you cannot receive their messages. To fix this, you have to find the option in WhatsApp to delete your account.
Tally:
Signal 1 WhatsApp 0
Is (b) achievable by all users who have this concern?