The incentive is to increase usage generally, with some other users paying. And to show people some small level of service, hoping they will upgrade.
Receiving phone calls is free, and the first phone I had (in 1999/2000 or so) was only used for my mum to call me, not the other way round. But that was still income for the phone company.
Around here you could even get plans where the company paid you for your incoming calls. One of those plans was so good that people sold their SIM cards years after the plan had been discontinued for new users (mobile companies are not allowed to cancel plans for existing users without their consent).
Receiving phone calls is free, and the first phone I had (in 1999/2000 or so) was only used for my mum to call me, not the other way round. But that was still income for the phone company.