I believe it has always been this way? Scammers use this trick a lot even today.
If JB faked the call from WHF he would have had to terminate the call before he left?
Yes, if you're correct, that seems to be the logical implication of it as we cannot otherwise explain his phone calls to Julie and the police at a point when he would not still be at the farm.
What it also means is that when terminating the call at the farmhouse end, he has to leave the handset off, so that he can get an engaged tone at the other end when making a potentially traceable call back to Nevill (and also so that it will look like somebody has used that specific phone).
This means we come full circle. The problem is that he has to be seen (or potentially seen) coming out of Bourtree Cottage at 3.37 a.m., or whatever the time was. At the same time, if you're correct, this means we also have another reason he can't use an answerphone as an aid because you still need somebody to terminate the call at the farm end.
This means, if he is guilty, and if we assume he believes that there could be a trace of calls somewhere (which seems a reasonable assumption to make), then the only way he can do this is by staging a call (making one up won't be enough), which in turn means he has to make the call from the farmhouse to Bourtree Cottage, then immediately terminate it and leave the handset off.
This leaves him with only one option, which is to take the bike back with him to Bourtree Cottage, just as the police say.
A flaw in this is the follow-up call to Nevill. I accept that Jeremy would get the engaged tone when ringing back, but there will be a 10 to 15 minute gap - at minimum.
What I think we can conclude is that if you have got the technical telephony parts of this right, then the bike was actually crucial to his plans, but it rests on the bike getting him back to Bourtree Cottage reasonably swiftly - say, within 10 to 15 minutes, and even then he is leaving a suspicious gap. I do find it very hard to believe he would use a ladies push bike over farm tracks, and in the dark too, probably without lights.
It does raise the whole question of why stage these phone calls? Why not just take the risk of questions about Nevill's body being found in the kitchen? He is creating a rod for his own back.