Can my business legally sell my landline phone number to a purchaser of the business

Please clarify your question as to location and how the business and number are connected ?

Not in the USA... They might stay with you as long as YOU hold the account, and keep it current... but the phone company owns the number and you don't, and will reassign them eventually after the account is closed out.

Sell, no, but you can transfer it with the business.

All you have to do is sign an agreement with your phone company. The last time I did this I had to agree to be liable for any charges incurred by the new party if they didn't pay them. But that was 40 years ago.