The answer to this question may depend more on the company's point of view than the technological issues. In my organization the phone and number is transferred to the new individual in the position so that clients calling from old business cards or other contact information is answered by the person responsible for that workload.
If you can get a release letter from the company, including an appropriate contact person, you may be able to port the number, but I would expect a number of hoops to hop through.
It may also be easier to transfer it to a new business account, than a personal account.
The situation you describe is one of the reasons I maintained a personal cell phone for non-business use.