Sorry to see that your question has gone unanswered.
Perhaps I could offer some advice if it is not too late.
The number which you wish to preserve is more than likely subject to exchange embedding, which means that if you wish to "move" the number to a premises outside of its current exchange, you will not be able to do so without raising a bespoke purchase order, requesting that process. This would be very expensive and most likely prohibitive.
If you are within the same exchange area then the process should be a simply one. However you strictly have to be the contracted party to make any significant changes to a land line number under telecoms regulations. BT have no obligation to enter into discussions about legal contracts with any other party than the original account holder, as named on the bill itself. If you have purchased the company as named on the bill itself, you have no legal right to the number, and BT can "lose" the number as a result of any changes you are able to make without any consequence to themselves.
One way round this is to gain the cooperation of the original payee or bill payer and register the account number for online billing services. You can then nominate a name change by defacto presumption, as the username and password holder. Having done this you could then make whatever changes, in writing or verbally with BT in the normal way, as well as ascertain the limitations which may apply, as set out above.
BT have no interest in your business welfare or the benefit you may derive from having taken over another business. Its a mistaken notion which many have fallen foul off.
In such a circumstance as this it would be better for you to have the original number dynamically forwarded to another land line number and continue to pay for this service on an ongoing basis. To do this you should only need the original account number on the bill and either explicit or else inferred permission of the original bill payer named on the original bill. Any bill would have this information on it.
As far as having the name changed this would also run into difficulties for the same legal reason. You are not the contracted party even if you are paying the bill. Unless the contracted party himself/herself initiates the changes, and does so on your expressed behalf, as an interested party, you could be left high and dry. Other posts on this forum elucidate some of the difficulties others have had in this area, even when they have been the contracted party.
BT appear to have a philosophy of talking good business sense but in reality avoid any possible conflict of interest where an established contract has precedence over gifting or else exchanging commercial interest, invested in a unique telephone number.
You may be able to use the One Bill service to append a company name to all of your BT services and this would suffice for accounting purposes. The likelihood of achieving a straight forward take over of an existing number which has not been offered into the pool of numbers available for business use, is slim at best. From what I have read here and in other places, as well as my own dealings with BT, I would not place any confidence in the likelihood of them acting in a strict commercial interest to your benefit. They simply don't care.
