Or the better Headline: How to become a askozia supported TIPTEL IP-Phone with YEALINK Firmware
Tiptel shows us the way: http://www.partner.tiptel.nl/eol/yealink.php?lang=de
In case of Internet Links are not for ever, here the copy & paste Instructions:
yealink T2xP IP Phones
As of december 2015 it is not longer possible for the Yealink T2xP IP phone series (with tiptel OEM firmware in it), to use our update server due to low certificate encryption used by Yealink.
To make it possible for you to change our firmware to original Yealink firmware, you need to change the update URL in the phone.
Please read the procedure carefully on how you can proceed this, and remember that this only works if the phones are running x.70.13.9 firmware.
The first digit in the firmware is telling you which phone it is:
9 = Yealink T20P
7 = Yealink T22P
6 = Yealink T26P
2 = Yealink T28P
Follow the next procedure to update the phones:
Reset the phone back to factory settings in the webinterface, or press the OK button for a while.
Change the update url to http://update.tiptel.nl/yealink/
under Upgrade and Advanced.
Clear the fields for User Name and Password (make them empty), and click on the Confirm button.
Go back to the basic menu and click on the reboot button to restart the phone, or restart the phone by powercycling it.
After the restart, the phone will collect data through provisioning and restart.
After this reboot you can update through the provisioning code for Yealink original firmware.
The provisioning code for updating is:
#*8845# + OK Update to orginal Yealink firmware V72
The unit will now update the firmware via provisioning.
After the update, there are some limitations: Tiptel cannot provide support on the given procedure and firmware that is loaded into the device
After a reset, with an original device for Yealink updated firmware, the Dutch language is no longer present
With the loaded V72 firmware you can load every firmware in the phone found on the Yealink website.
I flashed today 5pcs Tiptel IP286 to YEALINK SIP-T28P with Firmware 18.104.22.168