Hi there Geffers, and welcome to Plusnet!
Unfortunately there isn't an app for that, but if you need your calls to come through to you when you're not at home, we do offer the Call Divert feature for £2.50 per month. I hope that this helps!

and if you are diverting to a mobile you will be charged for the call I believe

It depends on which call package you have with us. Whilst call divert is on, you would be charged for incoming calls on the same basis as you are charged for outgoing ones on whichever call plan you're on.
So for example, if you're on Anytime with Mobile bolt-on and were diverting to a mobile, you'd only be charged for your diverted calls after the first 100 minutes had been used up (or if a call went over 60 minutes), it that makes sense