provided by Star2Billing S.L.

This isn't a bug so much as a defect.

When I change the vat rate, it should not apply to invoices prior to the change.

I tried printing old invoices and the new VAT rate applied. When an invoice is generated, the VAT rate inforce at that time should be applied. Guess this needs to be stored in the invoice header table.

regards

That's true.
Another related problem is that calls made by customers using a currency other than base_currency are shown using the current exchange rate, rather than the prevailing rate at the time the call was made.