When sending and receiving Signal, iMessage and WhatsApp messages, Beeper’s web service acts as a relay. For example, if you send a message from Beeper to a friend on WhatsApp, the message is encrypted on your Beeper client, sent to the Beeper web service, which decrypts and re-encrypts the message with WhatsApp’s proprietary encryption protocol.
@Display [email protected] is right, confirmed via their official FAQs: https://www.beeper.com/faq#how-does-beeper-connect-to-encrypted-chat-networks-like-imessage-signal-whatsapp