Jeremy made up the call from his father it never existed he made up this call because he wanted to make up an alibi.
Where is the proof? Where is the proof that a call did not exist? Where is the proof it did?
It can't be proved either way and that is a fact an arguable fact that will go on and on....but what would a jury of today think? 
The proof he made it up in order of importance is:
1) Evidence proves Sheila didn't load the gun, fire the gun or beat anyone with the gun and was murdered along with everyone else therefore Nevill would nto have had any reason to call Jeremy claiming the things Jeremy claims
2) Nevill had no opportunity to make a phone call to anyone, the killer entered the master bedroom and fired a full magazine at June and Nevill. Because Jeremy removed the phone from the phone from the bedroom and placed it in the kitchen (for no valid reason since the kitchen phone was not broken like he claimed) there was no phone for Nevill to use before being shot. His jaw/throat injury prevented him from being able to speak if he phoned someone.
3) Jeremy's actions are totally inconsistent with the actions of someone receiving the call he alleges to have received. Someone in his place would have either rushed over or dialed 999, he did neither he called Julie (some roommates place the timing of that call before he even claims Nevill phoned him) and then wasted time looking up police numbers.
4) Julie said Jeremy told her in advance of the murders he planned to pretend he received a call from WHF.
5) If Sheila had gotten the gun Nevill would have disarmed her or armed himself to oppose her or if too scared to do either would have phoned 999 not his son.