Be aware that it isn't just a Plusnet problem, it can happen with any MVNO (Mobile Virtual Network Operators) - there are only 4 MNOs (Mobile Network Operators) in the UK - EE, O2, Vodafone and 3. All the others (BT, Plusnet, GiffGaff, Tesco, Virgin etc.) 'buy' usage of one of these networks and sell it on. As I understand it none of the MVNOs (possibly with the recent exception of Virgin - but I'm not 100% sure) pay for voice-over-4G. So when you have 4G on, and your 'phone gets a call, it has to switch to 3G to receive the call.
In my case you could see the 'phone switch to 3G but it didn't ring - the supposition is that the 'phone isn't switching fast enough for the network/base station which thinks it's not getting a reponse and diverts the call to voicemail. It only happens in some areas, so maybe it's a base station configuration issue which would be up to the MNO, but of course the MVNO has to convince them of the problem. If you Google 'calls go straight to voicemail 4G 3G' you will find this problem afflicting users of various MVNOs.
I can see that this is a tricky situation for the MVNOs, but at the end of the day if I am sold a 4G service I expect to be able to use a 4G service and receive 'phone calls. As a Plusnet broadband customer I was lured to their mobile service by the very competitive pricing, but at the end of the day it didn't work so a lot of time and a little money was wasted by 2 successive changes of provider and number porting delays.
As I said, this might not be the problem in your case.
