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Ordinarily, a premises is not responsible for lost property of customers. However, if you can prove they actually had the phone in their possession
and then lost it again, then this would be grounds to seek to hold them liable for the damages as they were a gratuitous bailee who had to exercise ordinary care to preserve your property for you. The difficulty in this case would be proving they actually did have the phone or that they did find the phone as all you have is someone on the phone saying they had the phone (and the chances of you getting that person to admit that in court might be very tough). However, presuming you can actually prove they had the phone, you can seek to not only hold them liable for losing it again and you can also file a theft complaint as it is likely someone stole it from the gym as they likely failed to take ordinary care to preserve the phone for you.
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