Flight crew over the South Pacific reports inadvertently accepting a clearance to FL380 via CPDLC. The crew is alerted when ATC queries them about their altitude; 18 minutes after the original clearance.

Narrative

At FL370 we received a CPDLC [Controller-pilot data link communications] ATC message asking when we could accept a clearance to FL380. I free texted back stating 'now'. Shortly afterwards we received an ATC message. I pushed the ATC prompt on the FMC CDU once as witnessed by the other first officer. The message stated 'WILCO'. There was no print or accept prompt and it stated page 1 of 1. After a brief discussion; we decided it was sent in error; and the only way to not have an open log item was to send it; so we did. Sometime later; we received a CPDLC message asking 'when do you expect to be level at FL380; you were cleared 18 minutes ago'. At this time we sent a message back requesting a clearance to FL380. We were then sent a CPDLC clearance stating; 'climb and maintain FL380; report level; 2nd clearance'. Upon reviewing the log; we saw the initial clearance to FL380 at 1524z followed by a WILCO. Upon reviewing later climb clearances; we concluded that the CPDLC message jumped ahead 1 page without our input; as if the accept prompt had been pushed on page 1. We never received page 1 in the original clearance to climb and therefore did not push the accept prompt. In retrospect; we should have immediately gone back to review the log and we would have found the clearance. Fatigue was not a factor as I slept solid in the bunk and had a few cups of coffee prior to the event. A maintenance write-up on CPDLC was sent thereafter.