I am in the process of cancelling my vdsl connection with 2 Degrees, which I have had since March 2019. I raised with the helpdesk the possibility of keeping the Fritz!box modem [7560] and referred them to their 2018 Geekzone posting confirming that although they own the modem, customers would not be charged for non-return if they have leased the modem for over 36 months. The 2018 post is number #2069210 at https://www.geekzone.co.nz/forums.asp?forumid=81&topicid=237484
Tech Support at 2 Degrees responded today to say the criterion is whether or not the modem is in or out of warranty and that my modem [4 years 3 months] is still under warranty and has to be returned.
Geekzone readers should note that the 2018 2 Degrees posting about 36 months is out of date. Presumably warranty periods vary with the particular model of modem.