"Call override services do not require you to sign up to a contract."
but services like 18185 (or 1899, which I use) can be blocked when the company handling line rental and calls is other than BT. Sky, TalkTalk and others can (at their discretion) block access to 'indirect services' and do
I'm using Primus (line rental under 9 quid) and 1899 for calls (5p per call to 01/02/03 numbers, around 10p/min to mobiles, but I use a mobile for those).
Sorry OP, if none of the networks provide a good signal, that's the source of your problem (along with cost from Sky). As someone else said, calls between DS and his dad are something for his dad to cover, if necessary with a SIM-only deal from Tesco (uses O2), O2, or one of the other mobile networks.
I get 600 minutes a month on a 15 quid/month deal from Three (it's their "SIM 600" on a 12 month contract, giving 600 minutes, 3000 texts and unlimited internet), but there are cheaper deals for just voice and text, and of course even texts to your landline will work - if you have a cordless phone that can accept SMS or if not, there's a text-to-voice service so DS's dad can type in the message and it will be spoken on the phone at your end.