since we have been waiting

The receptionist has already taken eight phone calls since we have been waiting in the lobby.

Please tell me why there is the Present Perfect Continuous tense after ‘‘since’’ in the above sentence, I know what it means, but should I imagine the expression ,since we have been waiting’’ as: ,since the beginning of waiting in the lobby till the last moment or before the last moment or this current moment’’?

Thank you very much

It’s talking about the period of time starting when the waiting began and continuing up until the present (i.e. they are still waiting).