This seems like more of an issue with working with an external client than with working remotely. Within a company you can generally just ping someone on slack

This seems like more of an issue with working with an external client than with working remotely. Within a company you can generally just ping someone on slack (or whatever messaging your company uses) for a quick 5-minute video call. For colleagues I work closely with, I will also just call them unannounced. The latter could potentially work with external clients (perhaps using their actual phone number rather than a video call) if it was arranged in advance. i.e. you have a kick-off meeting where you discuss how much contact the client wants and what kinds of questions should lead to immediate communication vs. slower async communication.