Why are you not sending text messages to each other if it is so hard to understand? You can have plenty of error correction to make sure the message always is correctly received.
If they are in a situation in which they can send W3W coordinates by text message, then they are in a situation in which they can send ordinary numerical coordinates by text message.
And I'm implying that there are situations where they cannot take the luxury of communicating by text with the extreme care required not to make the slightest typo in W3W.