When buying or selling a business, what is and isn’t included is up to the parties to negotiate. So, (almost) anything goes. It can also depend on what you are actually selling.
When you sell your business, you are essentially selling everything that contributes to the revenues and profits of the business. If your mobile phone number has been used in the operation of the business (and therefore contributing to the revenue and profits), then this would generally be transferred in the sale. A good test is whether the number is on any business cards, your website, or other contact material. The buyer wants all points of contact to go to them.
In most cases, what the buyer is after is your mobile phone number. They probably don’t want the phone. I suggest you agree to hand over the number and erase the phone. It gives you a good reason to contact people with your new number – and let them know you have sold the business.