It’s illegal to use a mobile phone whilst driving your car so what do you think the law is for when you’re using a forklift? Well, whilst it’s not illegal (unless you’re on a public road) there are some serious health and safety issues to consider.
A risk assessment should be undertaken to identify what those issues are and the biggest factor will be distraction, much the same reason why phones are banned whilst driving. Yes, you could use a hands-free kit but forklifts are some very hefty pieces of machinery and in a warehouse there will be higher risks.
If you do need to operators to keep in contact and communicate then there are safer communications devices you can install in your warehouse.
There are other ways for driving to get distracted too and that could be an unsecured item, in this case a mobile phone, falling within the cab area and the driving making a reflex grab for it and losing control.
Your risk assessment will identify this and any other issues and as management you will need to make these rules and protect all your workers.