Hi Andrew
First go to Settings > Organisation > Details > Tax > Manage VAT schemes - from there you will need to make sure the date you want to create the bill is covered (not VAT registered presumably).
Next you will create the bills as usual (without VAT).
Finally, you will do a journal entry to reclaim anything prior to registration by heading to Tools > Journals > Create. You can do a single bulk journal on the day you registered to cover all items. I would've thought it'll look something like this:
Line 1 - this is essentially reversing the P&L entry you've made when originally entering the bill i.e you've spent £15.53 on telephone costs that you're now removing.
Line 2 - this is you putting the telephone costs back into the P&L but at their net value of £12.94 - please remember to tick the VAT return purchase box over on the far right.
Line 3 - this puts the VAT element of the expense into the VAT control account ready to be reclaimed from HMRC - again, please remember to tick the VAT return purchase box over on the far right.
As I say, you don't need to do it bill by bill, you can just add them all up and do a single bulk entry (the only thing you might need to do is add a few extra lines if they relates to different P&L categories (such as office supplies, accountancy fees etc).
Sound ok?