Since 2016 Telefonica / O2 provides a free ADSL WiFi router with DECT telephony to its customers for free. The device is a ZyXEL OEM device, model number 6641.
The Telefonica / O2 product information does not give a hint that this device actually contains free and open source software. Our request to O2 to provide the source code or at least a link to it remained unanswered. Upon our insisting requests ZyXEL finally gave in and provided the following download link:
We have not yet checked the archives for completeness.