There is nearly always a SIM only tariff with the same features as the tariff that came with the 'free'/'subsidised' phone.
For my phone it cost me €140 to buy outright but the 18 month contract to get it 'free' was another €15 a month - and they'd not let me know when the 18 months was up...
Don't know of any service provider that reminds you on expiry - they probably should be made to, though. It'll never be retroactive so no chance of it being a refund frenzy like PPI.
I've got my electricity, broadband (+ not even plugged in landline) and mobile on 30 day, post-expiry setups...