First, we are sorry about your son's sentence. Twenty-five years is a long road and the first days after sentencing are among the hardest for everyone involved.
Here is what to expect in terms of communication. Kirkland Correctional Institution is South Carolina's reception and evaluation center, which means your son will go through an orientation period of about a week or so before full privileges open up. During that window phone access is limited while he gets processed, classified, and assigned to his permanent housing unit.
Once orientation is complete, he will be able to make calls through GTL's Connect Network, which is the phone carrier South Carolina DOC uses. The calls come from him to you, not the other way around. You cannot call him directly. To receive his calls you will need to set up an account with GTL or have funds available to accept calls on their system.
If you are outside South Carolina, InmateAid's discount phone service may be able to reduce your per-minute costs significantly by providing a number that forwards to your current phone. That is worth looking into before the calls start coming so you are not paying the full carrier rate from day one.
In the meantime, the best thing you can do right now is send him a letter. Mail reaches inmates even during orientation, and having something from you waiting for him in those first difficult days means more than it is easy to express. InmateAid can get a letter and photos to him at Kirkland quickly, with your home address kept off the envelope entirely.
He is going to need you to be his anchor through this. Start the communication as soon as possible.