Until your next bill is produced, your current month's plan cost and allowances are
worked out as follows:
Plan cost - The pro-rated totals from your old and new plans. You
pay your line rental in advance, so you'll get a credit in your next bill for the
extra line rental you've paid.
Allowances - The total allowances from your old plan, plus the
pro-rated total from your new plan.
The fact that we don't pro-rate your old allowances is good news, because it means
there's a good chance you'll end up with a few extra allowances until your next
billing period starts.