https://missiveapp.com/features/whatsapp-twitter-facebook-an... support a couple of channels. So does https://frontapp.com/integrations?filter=Channel and Intercom can be connected to other channels (https://zapier.com/apps/intercom/integrations/linkedin)

I'm not saying you couldn't also build one. But if your main goal is to do sales then you might not have to start a new company.