I doubt that a court would rule that a lien has been created by contract, or under common law, in respect of the phone and the keys. (A lien is the right of someone to hold onto property until they receive payment. e.g. a motor mechanic can refuse to give a car back to its owner until he's been paid for the repairs that he was asked to carry out but that's because there was a contract between the mechanic and the vehicle owner that specifically related to the car).
If your OH were to hold onto the phone and keys without having a lien though, then he's committed the tort of 'conversion'. However that's only a civil offence, not a criminal one, so the employer would have to sue your OH to get a court order for their return. (He couldn't be prosecuted for the criminal offence of theft, as the legal definition of theft requires that the offender must be seeking to permanently deprive the owner of his property which, clearly, he's not trying to do).
Getting property back from a former employee who's keeping hold of them can be far from easy. Just ask NatWest!
https://www.bbc.co.uk/news/uk-england-hampshire-59578954