According to the plusnet forums, it is their parent. BT, who is keen to keep plusnet as a broadband provider only. When I upgraded the plusnet rep suggested vonage but the trust pilot reviews were enough to convince me to go with the plusnet forum recommendation, A&A

I am with plusnet and moved from fttc to fttp a week ago. This meant I lost my copper line and, having read around, signed up with Andrews & Arnold to provide voip. I was advised by them to place my A&A order, including port request, on the Wednesday afternoon, in advance of the Friday fttp upgrade. This would lose enough to the upgrade to avoid accidentally terminating the plusnet contract in advance of the fttp upgrade. As it happened, I don’t like my existing landline number and instead chose a new number on the Andrews and Arnold website. My understanding is that my old landline can be ported within 30 days of the fttp upgrade, failing which it is lost.