The web version of whatsapp does not do calls, only messages. And when you have family/friends scattered around 3 continents, traditional PSTN calling is not an option.
About Signal: especially now that Whatsapp has adopt their encryption protocol, it is very hard for me to convince everyone else to switch to Signal, and AFAIK Signal does not do group messaging, so I would have to convince other people to switch to an inferior solution with no extra value to justify the tradeoff. If I had decent XMPP/SIP clients I could at least try to sell the extra value of having your communications outside of control of any single company.