An alternative answer. You just need to make sure the "=" is not the first character on the line, so adding a space in front would be enough to prevent the Heading format being invoked. But that obviously shifts the text by a character. So another option is to prefix with some formatting tag, <plain> works well as it adds no extra formatting. Unfortunately tags only apply to the current line and are ended by a carriage return (\n), so you can't just blindly wrap up all your text, you have to target a particular line:
Bob -> Alice : hello\n<plain>=123</plain>