For private businesses the management may decide what is allowed on the business premises and what activities are done there. While they do need to provide restroom access they don't have to have it designed to accept cell phone, wi-fi, or satellite communications. For most of my life there were no smart phones and we got along pretty well at home and at work without the need to have a phone in our hands all day. The employer is not obligated to provide the rest room phone access, particularly if the employer is not the owner of that space. Most offices today are leased and it is the landlord that is responsible for maintaining the building and common areas like restrooms. The employees need to understand that there are also significant privacy issues in allowing that access. Indeed, a smart employer today would prohibit even taking the phone to the restroom. There really is little need to have one there.
If the employees are that fearful of going to the bathroom without cell phone access and the employer or landlord won't provide it I see two options. The first is to form a union and collectively bargain with the employer over the matter. Depending on the make up of the workforce there that may or may not be a viable thing to do. It also carries risk that the employer would permanently replace you, effectively terminating the employee.
The other option is, of course, to quit and find a new job where they will the cell phone acess they want.
They may want to meet with a labor law attorney to find out the details of what they can do and what the employer may do in response before taking any action, particularly if they like the job they have other than this one issue.