This device?
https://assets.avm.de/files/docs/fritzbox/fritzbox-7530-ax/fritzbox-7530-ax_man_en_GB.pdf
Check the ADSL connection serving the Fritzbox.
Is there a splitter/filter on the incoming service line?
Something like the following line diagram:
ISP ---- ADSL cable ----> [IN] Splitter/Filter [OUT1] -----> Fritzbox and Splitter/Filter [OUT2] ----> Telephone system.
Splitter/Filter likely to be very small and could be placed anywhere along the incoming path.
Splitters/filters are made as cheaply as possible and can cause intermittent problems such as you describe. Could also be a faulty cable/plug as well.
FYI:
https://www.isat.co.za/support-online/pots-filter-basic-guide.aspx
You can google for other similar links and explanations.