Yes you could choose the number of rings when you set it up but I don't think you got a choice of more than 8 rings which was never long enough 
There is considerable evidence that Jeremy never received a phone call from Nevill. The issue of whether he would have been likely to even hear the phone and be able to answer in time wasn't even discussed much at trial because of there was so much evidence that the call never happened.
The phone was in the downstairs of the house while the bedroom was upstairs. They never tested how hard it was to hear the phone ringing while in the bedroom. If he were sleeping like a log as he claimed it would surely take several rings to wake up sufficiently to even recognize the phone was ringing. Depending on how loud the ring could be heard from his bedroom it might not even have been able to wake him up.
After waking up and appreciating it was ringing he would have to decide whether to try to answer it before the person calling gives up. If he had his answering machine on then potentially the person would be leaving a message and he could pick it up before the person hangs up. If it is off the person could decide to give up rather after a certain number of rings. The family said the stairs were narrow so he could not get down them too quickly and they believe that if Nevill were in a panic as claimed then after a certain number of rings with no one picking up he would call 999 not just leave the phone ringing and ringing until Jeremy finally got downstairs to pick it up.
In fact, knowing he might not hear the phone because it was downstairs and knowing even if he did wake up and hear it that it would take a minute to answer means he probably would not have wasted his time on such. If Sheila were running around with a gun he could not expect to have an unlimited time to hang out on the phone. Nor would he leave his wife and the kids in bed asleep with her wandering around with the gun as he wasted time on the phone. This is a huge problem. It makes zero sense to waste several minutes on a phone call to Jeremy then 10 plus more minutes wasted as Jeremy gets dressed and drives there and is let inside by Nevill so that Jeremy could try to disarm Sheila.
Nevill had the same physical ability to disarm Sheila as Jeremy if not superior ability because he was even larger than Jeremy and more importantly he had a calming effect on Sheila while Jeremy didn't so he had a superior capacity to talk her down. He had the NEED to do so ASAP the suggestion he would waste time calling Jeremy and then sit around with his thumb up his butt waiting for Jeremy to come do it is absurd. He would have tried disarming her himself not have called Jeremy.
Before calling anyone he would have locked his wife and the boys away where Sheila could not easily get to them. He would not have left her wandering around the house with the gun in a crazy state and did nothing to wake them up and warn them to hide.
The call is not credible at all even before looking at: Julie's testimony that he committed the murders and lied about receiving a call from Nevill; the evidence which demonstrates Sheila didn't load the gun, didn't fire the gun and didn't beat anyone with it so there is no reason why Nevill would have claimed she was running around with it; and the fact that after allegedly receiving this call Jeremy failed to react as someone who received such a call would have reacted. He didn't rush over nor did he call 999. He called Julie (some of the roommates say this call was earlier than the time he even claimed Nevill called him) and subsequently looked up various police station numbers. This not only conflicts with what someone actually in his place would do but conflicts with his own account which featured him claiming he tried to call back and the phone was busy so he then immediately called police. For his claim to be true someone had to hang up the phone in the middle of the call then after Jeremy hung up the person took the phone off the hook and left it there. This is another thing that makes no sense. In the meantime though he didn't call police he called Julie 10 or more minutes after calling Julie he finally called police.
Other problems are that Jeremy removed the phone from the bedroom to replace the perfectly functioning kitchen phone and lied about the kitchen phone being broken. The shooting commenced in the master bedroom with Nevill being shot 4 times and June 6. There was no phone to use prior to the shooting because Jeremy removed it. Nevill could not speak after this because of his injuries. It is not credible that Sheila marched Nevill to the bedroom to shoot both parents together. She would have shot Nevill upon finding him on the phone.
People biased in favor of Jeremy are willing to ignore anything damaging but if you look at all of this objectively it is very bad for Jeremy. It is hard coming up with anything to use to help Jeremy to rehabilitate his claims of receiving a phone call in the face of all this.