I can’t figure out if you are talking about the phone signal to a DECT phone or phones, or if it is the WiFi on the broadband that isn’t reaching your office.
You can get anybody to install extra phone extensions for you these days; they should be qualified and/or experienced to do the work, of course, but anything on your side of the Master Socket is up to you, and you don’t need Now or BT Openreach for this.
But you don’t need a second line, you just need an internal extension; and if you are talking about the reach of the broadband,, perhaps not even that, as an alternative router with a better reach, or a WiFi extension over PLAs, like this:-https://www.currys.co.uk/products/tplink-tlwpa7517-av1000-wifi-powerline-adapter-kit-twin-pack-10222...
might suit you better.
But what is it, exactly, that you are trying to achieve here?
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.
If you take an offer that comes with a ‘free trial’ of Boost or another Membership, Cancel them straight away. You will still get the full duration of the trial, and if you decide you do want to continue with one or another of the trial products, just revisit the Cancel, and UnCancel it.