EASA differentiates between airport standby, standby not at an airport (vulgo home standby) and reserve. The latter is actually quite interesting, as it requires the operator to call at least 10 hours in advance, but then has no credit for time on call. However, during each reserve day an 8 hour period has to be planned in which the operator cannot call the crew, to allow for a full sleep period. This does not have to be at night though....
Standby, whether at an airport (requires to be have a single room with adjustable temperature, lighting and a bed), or at home, will always count into the cumulative duty time limits. (7 days, 28 days etc), even if not called. Additionally the time of standby plus the duty resulting from a call out are not allowed to lead to an awake time of more than 18 hours. However, a certain time on home standby can be discounted as it is assumed that the crewmember sleeps anyway. But quite honestly, the EASA rules are not all that easy to understand and there do exist alternate means of compliance, not only those in the general means of compliance that EASA published.
All the regulations are available on the EASA homepage, and additionally there are FTL calculators, for example on the ECA webpage (
https://www.eurocockpit.be/sites/def....html#settings).