It's more likely to be 2.50 or 0.00 plus the infamous "line rental is required"
Ofcom and ASA are both mulling over the way consumers have been repeatedly hoodwinked by the adverts, with firms like BT having masses of small print in white text that's almost unreadable on TV, and similar stories for near everyone else.
Part of the reason for this 'requirement' is that the ISP (well, Telecommunications firm, really) is able to get a better deal, and save on their costs to some extent.
Openreach rents out the line.
Companies like EE, Sky, TalkTalk tie most customers into taking broadband and line rental for their deals. They have data equipment at the exchange and can therefore share their links back to London between all their customers at the exchange. They may pay Openreach for carrying their network traffic (voice and data) but they pay much less than if they had to pay Openreach for carrying the 'internet data' for them (Openreach would have to count it, log it, bill for it, add a profit margin)
The customer gets a (generally) cheaper price, (but sometimes the company doesn't rent enough capacity, so services might be a bit sluggish). The company locks in all your calls and can charge whatever per minute fees it wants.
My rental from Post Office Telecomms is handled by TalkTalk (whom I'd never touch direct!)
Plus.Net is on my other line, line rental from what was called Primus (now renamed Fuel Broadband, but I never opted for them to provide internet)...
This line rental is sold through Openreach as a 'BT Wholesale' product and as a result, I can use 1899 and 18185 codes to get cheap calls to Europe, Far East, etc.
Most of the firms which insist on their own line rental can inhibit the ability to use those short code numbers, so my Post Office (TalkTalk) line makes 1899 calls fail but I can use it to dial friends in USA for 1p/minute on my 'Primus' line (or on a BT line, or with some other firms, no doubt).