A probation officer used a defendednt cell phone while the defendant was incarcerated. He texted on the defendent's phone using the word "I." Is this a violation of the defendants rights?

No, it may be a violation of some evidence ( if phone was evidence) or some violation of property management. ** why did he have the phone to start with,

But you have no rights to protect you from your PO using your phone, he has the right to search it and use numbers in it to try and get more evidence against you. Most likely hoping someone would text back with things that could be used against the person.