I was led to understand the same with Telenor and permanent residency but it's not actually true. The following is an extract from an email their customer service team sent to me:
All persons who are not citizens of the Republic of Serbia and hold a residence permit for one-year period are eligible to enter into a contract/annex without purchasing a device.
In the even that you choose this option, documentation you need to provide includes the Temporary residence permit issued by Administration for Foreigners of the competent Police Directorate at the Ministry of Interior of the Republic of Serbia with validity no less than 6 months, and the travel document that contains registration number.
In my case, although I had less than 6 months remaining on my temporary residents visa, as I have been here almost 5 years and had all my previous visas, they were willing to make an exception.