The first call ends at eleven and the next starts at eleven. The calendar fits both, but you still need to capture notes and open the next set of documents. A buffer is not necessarily unused time. It makes room for work that otherwise has to happen invisibly between two appointments.
Describe the transition
For the next pair of appointments, ask what must happen between them. Do you need to change rooms, record a decision, send a file, or simply switch connections? Two short calls at the same desk may need little transition time. A visit at another location needs a different plan. Estimate the gap from those actual steps instead of assigning the same duration everywhere.
Try observing a few transitions deliberately. There is no need to time every second; note which step repeatedly gets squeezed out. Perhaps the conversation is not too long, but the follow-up is too substantial for the gap. In that case, give follow-up its own work block while keeping a smaller technical transition buffer between the calls.
Check what automatic buffers cover
Some appointment-booking tools can leave buffer time between bookable slots. Google Calendar describes this in its appointment-schedule settings. Check which events the rule actually covers. A booking page may protect its own bookings without moving every manually created event. Account type, selected calendars, and configuration can affect the result.
If the application has no suitable feature, add a separate block for an important transition. Give it a recognizable purpose, such as Prepare for project call. When the associated appointment moves or is canceled, review that block as well. An abandoned buffer can otherwise make useful time look unavailable after its original purpose has disappeared.
Do not quietly spend the gap twice
A free-looking quarter hour can tempt you to add another task. If it is already needed for notes and a room change, it is not also available for another conversation. Notice what you actually put into the gap. When the whole sequence repeatedly overruns, adjust the duration, scope, or order of the appointments. Adding a buffer alone will not fix a meeting whose agenda consistently exceeds the time agreed for it.