The trading day will not close while sales are still held on the phone. Closing over the top of them is how a sale ends up counted on the wrong day. Send the held sales first, then close.
What blocks the close
Almost everything held blocks the close: sales waiting to post, sales flagged for review, sales whose day was closed, and sales with a clock problem. There are two exceptions: a gift-card row and a quotation row. Both are things that could never have been captured offline.
A held row that can no longer post still blocks the close. A reused reference or a used card slip is an example. Money that was taken has to be dealt with before the drawer closes. The way out is a manager discard, and it is offered from the review sheet, not from the end-of-day screen where the block appears. A manager discard leaves an audit record only for a sale that was composed offline.
The check is per phone and per operator
The close looks only at the phone you are closing on, and only at your own held sales on it. It cannot see another phone's held sales, and it does not see a colleague's held sales on the same phone.
Because of this, every phone and every operator that sold through the outage has to send their held sales before the day closes. Sending the phones alone is not enough: a second person's held sales sit on the phone and only that person can send them. A sale still held on a different phone does not block this close. It is caught later, when that phone sends its sales, as a sale whose trading day was closed while it was away.