Did you go into the phone and see if their is a code for flash?

Also, not sure which ESI system you are using this on, but did you look to see if ESI has a code for flash, you may want to look at the codes for analog ports.

Lastly, why did you use a Yealink phone? Seems to be the client would lose a lot of functionality.

Pretty cool though you got it to work.

Walter