I guess we're locked into treating this as a question about rules and policy rather than as a technical question. Too bad.
Let's address the "technical" for a bit, since the moral has been well covered.
In all likelihood, the employer doesn't "own" the SIM. The SIM is probably the physical property of the cellular provider. The employer is paying the cellular provider for service, the SIM is the "equipment" provided to the employer for accessing that service. The service plan is what the employer has the right to control.
The cellular provider will know what IMEI is associated with the SIM. The employer might ask for that information if they felt the need to confirm that they were paying for service on company-sanctioned devices (concerns about stolen SIMs). (Company-sanctioned devices would be either company-owned or employee-owned bring-your-own devices that have been registered with the employer.) I wouldn't know whether the cellular provider is allowed to supply that information to the subscriber (employer). The cellular provider might even offer device management as a service to their business customers. All of this may vary from country to country.
The prudent thing for the employer to do from a financial perspective is to cancel service on devices that cannot be identified as legitimate. How many employers actually do this, I have no idea.
As to whether a cellular provider or their customer can determine who is in possession of that "incorrect" IMEI/using that SIM without authorization? That would be very challenging - something likely requiring a police investigation. Chances are, an employer would simply pull the plug to end further theft, rather than try to identify the thief.
Is a business in a position to demand access to the
data that travels over the cellular channel they pay for? Not to my knowledge. Well, if the data passes through the employer's VPN they would have the right and ability, but for data traveling over a public communications network, no way.