As for alternatives to WhatsApp, it would need to be something that wouldn't get a Chinese person in Hong Kong (and who often travels back to mainland China) into trouble just for having on their phone. Things like WeChat, Skype and Zoom are acceptable to the mainland authorities, WhatsApp is excusable for someone working In HK, but things like Signal are out of the question. Just having it on their phone could make that person a target for investigation/punishment.