Agreed, the issue will be the voice line rental, and how SYDR get around that, the £8 ish a month for MPF could prove expensive if people don't use the STB much at all, then add on the cost recovery from their network.
I suspect the type of household who are payg mobile and no internet, will have also dumped the telephone line already to save the £12 or so a month for the line rental. Those on the BT social tariffs may have retained it.
In my opinion they would be better off doing a very basic package at say £7 a month which included access to NHS/local gov websites, with a simple 0.5GB allowance, and provide a STB that connects to the TV. Then people would have option to browse other sites within allowance, and top up usage as they wish.
Plusnet (local to SYDR) already offer very cheap broadband in Market 3 areas, so if people aren't buying that, then price of the connection is not the issue, but the thing is using a computer.