"you'll need a residential line and a BT calls and phone line service package from BT before we can install BT Infinity."
Really? I had a call and phone line package from IDNET when I ordered fibre. So, if you want the Infinity product you need to move back to BT for calls? Many other fibre over BT suppliers out there.
I think you've overlooked the point that "BT Infinity" is a specific BT Retail branded product, not to be confused with the BT Wholesale FTTC product resold by independent ISPs.
BT Retail want customers to take their phone service as a condition of supplying their branded Infinity product. If you don't want to do that, then get a FTTC product from another supplier using the BT Wholesale product which doesn't require you to take BT's phone service, but it won't be BT Infinity.
This strategy won't work, however, with companies that take over your phone line entirely (i.e., MPF) unless they also happen to offer independent exchange-based (GEA) access to FTTC, such as TalkTalk which is the only one I know of at present. In that case, you are still tied into one supplier for all their telecoms services and also for an 18 month contract.
I hope that's clear? :^)
Every day, ...... is another day