If what you want is the absolute best value, which is odd given that you have been paying way more than necessary by using separate providers for (presumably) decades then Yes , contact whoever you want to use ,( beware not every broadband provider offers to bundle telephone services with their broadband ) , and let them arrange the switch , that’s if you want to have the same provider for both , just not BT , otherwise pick a separate Voip provider , purchase the extra equipment ( if it’s needed often a special VoIP phone , or ATA is required with these type of independent VoIP suppliers ) and then pick a standalone broadband provider and contact them both separately, have in mind this may result in a break in service if they don’t migrate you from BT and Plusnet in the correct order and with sympathetic timescales.
TBH , given the obvious resistance to change you have shown so far ( as witnessed by being on a shared metallic path facility product not actively marketed for decades) it’s a little weird to not simply take what’s on offer as it’s clearly the easiest thing to do , and it’s at a decent price , but that’s up to you.
