WHy don's Plus net use unallocated call mins to pay the costs incurred for calling Mob's and Premium numbers that way I would not need to keep topping up the credit amounts. We do not use the phone much and often have a surpless 700 mins unused which is lost yet I have to top up with £5.00 each month which on top of the Plusnet fee is a lot..
I can't comment specifically, but this will be down to their business model.
The cost of providing off-peak landline calls is significantly lower and this is why they can afford to give away so many with the subscription. The model is probably also set up on the expectation that most users won't use the full allowance, so using surplus minutes to contribute towards (significantly more expensive) mobile and premium rate calls would almost certainly lead to a significant subscription increase.
In fairness all telecoms providers operate in exactly the same with, with the sole exception (AFAIK) of the T-Mobile Flext packages that simply give you a call allowance to spend as you wish.