A small business starts, and the number it gives out is the founder's own mobile. This is entirely sensible at the time - it exists, it costs nothing, and the founder is the business. The number goes on the van, on the cards, on the website, into the directories, into the phones of every customer and every supplier. Years pass, the business grows, other people join, and the number is still the same personal mobile, now functioning as the main line of a real company. Nobody ever decided this; it simply never came up, because nothing about it ever caused a problem.
The difficulty arrives when the business and the person need to separate, and there are more ways for that to happen than people expect. An employee whose personal number became the number customers ring for a particular service leaves, and takes it with them, along with every customer who has it saved. A founder wants to sell the business and discovers that a substantial part of what a buyer is paying for - the stream of calls from people who already know the number - cannot actually be transferred. A partnership dissolves. Somebody wants, quite reasonably, to stop receiving work calls at ten in the evening and finds there is no way to do so without disconnecting the business.
There is a smaller, more constant cost too, which is that a personal number cannot be shared. It rings on one handset, in one pocket. It cannot be answered by whoever is free, cannot ring in the office and on a mobile at once, cannot be picked up by a second person when the first is on a roof or under a vehicle, and produces no record anybody else can see. Calls are missed not because the business is too busy but because one specific person was. For a business whose work arrives by phone, that is a direct and continuous loss that never appears in any account.
The fix is unexciting and cheap: get a number that belongs to the business. Ordinary business phone services now cost very little, run over the internet rather than requiring a line, ring on several devices at once, can be answered by whoever is available, and - the point of the exercise - stay with the business regardless of who works there. The practical move for a business already trading on a personal number is a gradual one: get the new number, put it on everything going out from now, and forward the old number to it for a long period while it fades. The old number never has to be taken from anybody. It simply stops being the only way in.