Who makes the line calls when there's no umpire on court?

You do, for your side of the net (Regulation 29, LTA Competition Regulations 2026):

• Each player or pair calls all balls on their own side, honestly and to the best of their ability.
• Call "out" and "fault" promptly after the ball bounces, loudly enough for your opponent to hear.
• If in doubt, the ball is good. Give your opponent the benefit of the doubt.
• The server calls the score before each first serve, loudly enough for the opponent to hear.

If you call a ball out and then realise it was good: the point is replayed, unless it was a clear winner (you couldn't have returned it), in which case you lose the point. If you've already made one wrong call earlier in the match, every further wrong call loses the point.

Other calls:

• Lets: both players are equally responsible.
• Not-ups and foul shots: called by the player on whose side it happened.
• Foot faults: can only be called by an on-court official, not by your opponent.

Clay courts (and artificial clay if the Referee approves): you can ask to see the ball mark on a point-ending shot, and may cross the net to look. If you rub out the mark, you concede the point. On clay, a wrong "out" call always loses the point.

Disputes: call the Referee or court supervisor. Spectators, parents and coaches must not get involved in resolving the issue.