Template BLF Variables

[Yisroel_MongoTEL]

That's a question for the system creating the templates, what variables it supports.

The phones themselves (what Yealink is concerned) need to get it already with the correct extension in the place.

You'd better be helped in the FreePBX forums, but I'd assume that if you can use __line1ext__ for other models (using the same template generation) you can use it for the Yealink phones in the same way.