What version of FreePBX are you running on your system and what firmware version is on your Yealink phones. This discussion in FreePBX seems to indicate there is an issue both with firmware version on the phones and also with older FreePBX versions. Apparently, FreePBX 16 solves the issue.
Also, you may have to put the parking slot extension 11 in the “Extension” field on the BLF key.