I think it is quite common nowadays to receive spam or suspicious calls.

My principle is simple: if the call is important, the caller will try again. If they don’t call back, they will usually text. If they do neither, then there is no real reason for us to return the call.

From my experience, genuine callers always follow up:

TM contractors will call repeatedly until they reach me before coming to my house to resolve a technical fault.

Parcel delivery personnel will call first; if I don’t answer, they send a photo or message about the delivery.

Work-related contacts given by colleagues will introduce themselves via text if I miss their call, and mention how they obtained my number.

Family friends who couldn’t reach me would leave a voice message explaining the issue and request a callback.

Since legitimate callers consistently demonstrate these behaviors, I don’t see any reason to feel obligated—or guilty—about ignoring unregistered missed calls. In my opinion, such calls can be safely disregarded.