Customer services phone-line was useless at sorting out my lack of voicemail service since porting from EE in March 2017. They didn't know how to fix and suggested I look myself at the voicemail settings.
A quick look on the community and a comprehensive response from Rebeka Preston to another user's similar problem (exact same I might add, suggesting Plusnet could fix this) and hey-presto, fixed.
If you are porting with an iPhone from EE to Plusnet then it seems that voicemail will not work until you manually intervene and type in some fancy code yourself. Plusnet should do this for us as part of the port process. Nobody tells you that your voicemail won't work, until you don't get that message about a job interview that a recruiter "left on your voicemail" but for which you received no notification, no missed calls.....
But, anyway, give that Rebekah a performance bonus. She seems to be one of the very few helpful staff.