An inmate number means someone has been processed into a correctional system, but it does not automatically confirm they are still inside right now. The number gets assigned at intake and stays on record permanently, even after release. Whether your friend is currently active in custody depends on what the record shows alongside that number.
If the information you found shows an active facility assignment, a current housing location, or no release date, that is a strong indication they are still in. If it shows a release date that has already passed, they may be out. Inmate locators on state and federal DOC websites typically show current status, so cross-referencing that number there will give you a clearer picture.
On your other questions, yes to all of them. You can send a letter through InmateAid right now without your home address appearing on the envelope. If he wants to write back he can do so through regular postal mail using a stamp, and your contact information inside the letter gives him what he needs to reach you.
For calls, he initiates everything. You cannot call him directly. If you include your phone number in the letter and he has funds on his account, he can call you from the facility's phone system. Collect calls run about $15 each without a discount service in place. If you set up an InmateAid phone account the rate drops significantly depending on the facility, which makes staying in regular contact a lot more affordable over time.
If you are intending to speak often, we would recommend you look into our Discount Telephone Service, it'll save you both a lot of money.