Before you make any calls or contact the facility, stop and think carefully about your own exposure here. This is not a situation where being proactive and helpful is automatically in your interest.
If drugs were found on a stamp you mailed, your name and return address are already on that envelope. The facility has it. Depending on how seriously they pursue it, that information can be referred to local law enforcement or federal authorities. Mailing a controlled substance into a correctional facility is a felony in Oklahoma and under federal law. It does not matter whether you knew what was on the stamp or not. Intent is something prosecutors argue about after the fact.
The first call you make should be to an attorney, not to the jail. You need to understand your own legal position before you say anything to anyone connected to the facility, law enforcement, or even your friend's family members. Anything you say in an attempt to explain or defend yourself can be used to build a case against you. That is not a hypothetical. It happens.
If you genuinely had no knowledge of what was on that stamp, an attorney can help you document that and advise you on whether any voluntary cooperation would help or hurt your position. If there is any chance someone else prepared that envelope or had access to your mail before it went out, that is exactly the kind of detail your attorney needs to hear first.
Your friend will have his own process to deal with inside the facility. Your priority right now is making sure you are not the next person facing charges.