It depends entirely on the company's policy.
For example, I had a company-paid cell phone and account at a previous job. When I left, I took the number with me -- but only because the company decided to let me do so. (Now if I could just get my former employer's name to stop showing up on caller ID.)
There could be legitimate reasons for the company not to do so. For example, if people inside or outside the company expect to call that number and discuss company-confidential information, the company might not want let the employee keep the number. In that sense, it could be like an e-mail address.
Is this a reasonable request for the employee to make when leaving?
Yes, it's a perfectly reasonable request. And either "Yes" or "No" is a perfectly reasonable answer.