[Fists] How to break in?

[W2SR - Robert I. Block]

I'm one of the more bashful types who waits for a QSO to end before tail-ending
on of the stations.  But I'd also like to complete my CC and sometimes I find
that the dinner bell wins before I can get my QSO.  What is the etiquette for
breaking in?

This afternoon, for example, Fred was having a FB QSO with Don, KN0WCW.  Fred
was 599+ here and Don was 589.  Now I already have KN0WCW in the log, but Fred
would have been a new one.  So I followed the QSO until the dinner bell won and
Fred signed CL.

Now it so happened, that during the QSO, some OM had an uncontrollable urge to
tune up (no call sign, maybe it was IM1LID?) and Fred paused and sent QRZ? to
invite him in.   In reflection, if I had send BK or some such, I might have been
invited and we could have exchanged RST and FISTS numbers and then I could have
either continued with the QSO or dropped out.

What is the right way to break in to a QSO and is jumping in and out after
exchanging a report welcomed or deprecated?

73
Bob Block
W2SR