"So lets summarise this as a new 'Refresher' - I am checking the below is correct not stating it is.
1) Buy phone at £19.99 on £28 airtime balance (referred to as monthly cost of £48)
2) Wait for topcashback (shows as £121 for me)
3) Cancel phone element when TCB pays out (about a month), this will then be £480 (24 months x £20), plus I guess one month for the contract £28. So this then means you have spent:-
£480 to pay off phone (which also then cancels the contract)
£28 for month you had an active contract
You then get
£100 HOF vouchers
so -£221 which makes the new total £307 if you take the HOF vouchers at face value.
What have I missed?"
So you pay one months rental, after receiving cashback call and cancel, clear the 24 month phone element of the contract phone in one hit.
This assumes cashback is paid from TopCashback etc but worth a punt.