Keep a journal of the fact that you made the calls, including date, time and what action you took - left message, spoke with ____, etc. Go to jail and try to check in. Bring newspaper and have friend take your picture in front of jail with newspaper date obvious in photo. Get name of intake deputy that refuses to admit you. Have name of good lawyer in your phone, ready to speed dial, if they issue a warrant claiming you did not show up. Keep trying to talk to the probation officer. If all else fails, retain a lawyer to assist you.
Mr. Case is correct. Document your attempts to do what you're supposed to do. I suggest also taking a video showing the date and time as you call the PO or go to the POs office, or when you go up to jail to try and check in. Any documentation to show you tried to do what you were supposed to will help.
Mr. Case's answer is right on. Go to the jail and try to turn yourself in regardless of having a special order. If you have something from the court - bring it. Bring anything you have in writing referencing the jail time requirement.
Be sure in the future not to wait until the last day to do whatever is being required of you. Yes, you "could" because that is within the time limit but as you can see from your situation, it may not be in your best interest.