How can we communicate short messages long-distance?

In our 5e game, we stole a boat. And a horse carriage. And a lot of other stuff for moving things between cities.

Our DM meant for this to be a way to skip road encounters. We decided to fund a shipping company and hire managers (Knights retainers, etc.) to turn a profit. The DM ruled that these would cover lifestyle expenses, provide us with easy smuggling, etc.

However, we want to keep tabs on the treasure we’re moving back and forth.

What can we use for communicating with our managers from anywhere in the world? What about from say, across the city?

We don’t need to communicate often or for long – a single round will work. All we need is for the manager to say “we need you back”