It's not so much a secret
1/ Get a list of the tariffs for each network - minutes/texts/cost
2/ map each tariff to each merchants name for that tariff, they're all different
3/ get a list of all mobiles
4/ map each model to each merchants description, again all different
5/ each deal consists of tariff + phone + phone cost (usually free) + gift + cashback + rental discount. Figure out how to map each merchants description of a deal into a standard format.
The hard part is every merchant has a different format in their feeds so the time is in figuring out how to match them all up. That done you need to update your system every 2 or 3 days as new models/tariffs are released - it's an ongoing job
This is why urmob/m4e are so popular. Nobody likes giving away traffic but it's probably cheaper than the alternative