As said, Bamber would -
Ring his answering machine from WHF. Leave a 10 second message. Hang up.
That ends the call. On both ends.
I thought of this as well. It's in my Jeremy scenario, albeit I agree that if he is going to stage the call in a way that is sensitive to timings (which is the only reason he would do it), then he must terminate the call at the farm end, leave the handset off the hook, and he absolutely must return to Bourtree Cottage by push bike (whatever the practicalities of this method of conveyance, another issue).
I had understood that an answerphone at Bourtree Cottage was examined by the police, but I don't recall where that is confirmed. Assuming that is the case, then it must be that Jeremy had a second answerphone, which he rigged up on the night and then hid.
Another issue here is, if Jeremy did have an answerphone at the cottage that was seized by the police, how did Jeremy take the call from Nevill in the first place? Surely if Nevill had first started speaking into the answerphone, that evidence would have been retained by police? Is Jeremy saying that he just didn't connect the answerphone that evening (they did have an on-off switch in those days, I seem to recall)?