Inmates make outgoing calls, not incoming ones. There is no phone line you can dial to reach someone inside a jail or prison directly. The communication only works one direction from inside out, which means your inmate calls you rather than the other way around.
To receive those calls, your number needs to be on their approved call list. The inmate builds and manages that list themselves, submitting numbers they want to be able to dial through whatever system the facility uses. Once a number is submitted and approved it becomes active for calls.
The list can be fairly broad. No requirement approved numbers belong to family members only, and friends, employers, attorneys, and others can all be added. The restrictions that do apply are specific and practical. Co-defendants in the same case are typically prohibited from communicating by court order. Judges, prosecutors, and victims connected to the case are also off limits. Beyond those categories, the inmate has significant latitude in who they add.
Some facilities cap the total number of approved numbers at a set limit, often somewhere between ten and thirty depending on the system. Federal facilities and state prisons handle this somewhat differently, but the inmate-controlled list model is standard across most of the country.
If you want to receive calls, let your inmate know your number and ask them to add it to their list. Once it is submitted and the carrier processes the approval, calls can come through. If you are also setting up an InmateAid number to reduce the per-call cost, that number goes through the same approval process and needs to be on the list before the savings kick in.