There are really two theoretical calls to consider here:
1) The alleged call from Father to Son saying "Help, sheila's gone mad and has a gun" (paraphrased)
The existence of THIS call is 100% paramount to JB's case. JB said it happened, but it's never been proven or disproven. If this call COULD be proven, JB goes free (in all likelihood).
The essential rationale for the Father to call Son is that the Father didn't want to get the police involved, and just wanted to 'keep it in the family' (entirely plausible, depending on the gravity of the situation inside the house at the time)
2) A possible call from Father to the police saying much the same thing as to the son in (1).
The potential for the existence of this call seems far less likely, although not impossible.
The rationale for this call (if it ever happened), would be some escalation in the gravity of the situation at the house, or impatience with Son not arriving at the house, so changed his mind and called the police?
The problem with any notion of this call is:
a) There's no real compelling evidence to say it happened. (although there are some curious activities regarding sending two cars to the scene of crime, but seemingly not at the same time)
b) There's no note of any 'esclation' of events at the farm
c) If events had escalated, 999 (and not local station) would arguably be the number called. (However, Father WAS a local magistrate and may have readily known the station number)
IF (2) ever happened, it's the same situation as (1) - JB walks free (in all likelihood).
What we DO know regarding the phone at the house was that a phone (in the kitchen) was off the hook, STRONGLY suggesting a) a call (or attempt) was made OR b) the killer took the phone off the hook in the kitchen to prevent anybody upstairs waking and calling for help OR c) the killer took it off the hook in order to corroborate the 'made a call' story or D) both b & c.
We are also led to believe that it was not possible to prove (via call logs/tests) if a call was made from WHF to anywhere, let alone a police station or the Son (in Goldhanger) - something which a number of people worry about - why WAS it so difficult to prove a call was or wasn't made? (accepting that call logging/billing wasn't as detailed as it is today, it ought not to have been rocket science either).
Finally there's a bit of confusion regarding the timing of the Son's call to the police, where someone 'misread' a digital clock by 10 minutes.
As an aside, the Son also made calls either side of the deaths to Julie Mugford the contents of which could only ever be revealed by Mugford.
Mugford who had originally given a statement to the police saying nothing bad about the Son later confessed that he had suggested prior to the deaths (the evening before) that he WAS going to carry out the killings, but Mugford had dismissed it as fanciful talk. She only went on to reveal that after the deaths, he called to say 'all was going well' (going to plan regarding the killings). The timing of the call after the deaths was ALSO difficult to pinpoint, but generally agreed it would be anywhere from 3.00 - 3.30am.
I think I've summarised it all - but could have a few mistakes in there. Others will be quick to point them out ! ;-)