Hi,
No, a boss does not have access to your personal phone and it would actually be illegal. He has access to consumption (calls, texts...) for example, via the bills. And he can have a say in case of abuse of personal calls during work hours.
See you later
Not to mention the security policy that can be managed remotely to allow or not the installation of certain applications, etc.
The ECHR had already made case law in this matter, after a dismissal that the employee contested as "abusive" .... ;-)
Let's imagine that you receive a MMS and save the photo; it is saved on the device and not on the SIM. ;-)