Give them a phone holding device that allows them to carry the phone around their neck so it can't fall out of their pocket or be left without somewhere to place it. (Men generally don't carry handbags!)
You could also have a policy stating that the cost to replace a mobile phone handset if it is lost will need to be borne by the employee and ask them to sign a form to acknowledge this when they are issued with their work phone. You would also need to have their written permission to deduct funds from their pay in instalments if need be. In some cases, you might pay for the loss of 1 phone but the technician who lost 3 phones in 3 months needs to be taught how to be more responsible for company property. He may not be so absent-minded if he knew he'd be up for covering the cost. Also, if as Qld
IR Consultant says, the phones may not have been lost at all.