Its possible, but needs closely monitoring. There are occasions when some lines do lose their phone no when coming from LLU to BT. Also depends on who currently owns the telephone no. Sky at one point lost most of theirs but things are much better now.
With this being a business then I imagine that telephone number is of vital importance so it would be best seeking re-assurance from BT that they will port the number. A new system came into effect a couple of months ago which supposedly lessens the chance of the number being lost. It is now the gaining isp (BTr) that will be responsible for handling everything.