A generic communication API (this might exist). When I want to communicate with my customer, it handles how they receive the message - whether that's SMS, Email, IM, slack, etc.
Not quite what you're asking for, but here is a service that provides the opposite side of that - all customer messages come into a single queue on your side and can then be processed by customer service and/or answered by a bot https://hubtype.com/
Front https://frontapp.com/ might be what youre looking for. If you want to do the plumbing yourself, Smooch is more at the API level.