I was not referring to the timings but that the operator reported the line as being engaged?
The GPO op, Jean Rowe, was just that a GPO op who provided a wit stat in her own words. She was not a trained lawyer and/or expert drafting a high quality report.
The line was open/engaged from NB calling JB. When NB heard SC open fire on June upstairs he left the phone on the counter thus the line was open/engaged. Jean Rowe was able to listen in on the line because it was open/engaged. Had someone tried calling WHF the line would have been engaged. You will note in AE's WS she says she tried phoning WHF on the morning of 7th Aug but does not tell us what sound she received

Had the line not been engaged/open I do not believe it would have been possible for Jean Rowe to listen in and/or put the call through to EP.