The law distinguishes between Bereitschaftsdienst and Rufbereitschaft. The latter does not count as working hours and is defined by being able to freely choose where you are and what you do during that time (until you get paged of course). That fits the typical developer oncall where you don't have to be on site or close by during your oncall shift.
If your employer chooses Rufbereitschaft as their on-call mode, then that means it all adds to your working time, and you can't exceed 10hours a day. Having a 2hour incident means no more overtime allowed. Additionally, they need to clock your work during each incident. Lastly, if you have an incident outside of working hours, you are not allowed to pick up work again for another 11 hours, unless it's another incident.
