I really do not think the police are concerned with your involvement in a conspiracy to purchase a controlled substance. In a criminal
case everything relevant may be presented. your phone call could place the suspect in a particular area, and show he was there with unlawful intent. Their word on tape not to charge you is not valid, police can lie to you in certain ways to trick you into waiving a right. I would request a signed document from the state attorney which explicitly states they are granting you immunity from prosecution for any crime you may admit to concerning an illegal controlled substance transaction or any intent or conspiracy to violate any laws concerning controlled substances. I really do not think the police would spend this time trying to get you on a drug charge. However you must be mindful that your phone is connected and they can force you to testify, you could evoke your 5th amendment right to not answer any questions concerning your violation of law, but the defense would have a field day with that on cross examination, which would make the prosecutor a little upset. you have absolutely nothing to lose by having an attorney present to make sure the document is in good legal order, and to make sure you are not tricked into waiving any right you may not even know you had. They will always say it is not necessary. They are very good at playing word games and mind games with people and getting them to confess without even knowing it. Most likely they just want to know what you will say if called to testify.
I would tell the police you want to help, and you appreciate their offer to state on the record that they are not trying to charge you. However you are aware that it is not their determination who gets prosecuted, and you would need an agreement from the state attorney before waiving your 5th amendment right against self incrimination. If they want to know what you know that bad, they can accomplish this. If you can afford to hire an attorney, I would. Other drugs are usually felonies and i would not mention that, as you don't even remember the call, but at the time you did occasionally use pot, and that would be the only reason for the call. Don't mention other drugs, it is irrelevant to what they need to know. If they say they need your statement first before they talk to the district attorney about immunity, or that it isn't necessary, inform them you will use your 5th amendment right to remain silent and not speaks to them, and if called to testify, you would also do the same and use your right not to incriminate yourself. The only evidence you committed a crime can come from you, the phone call itself is not proof of what was spoken, and to convict they would have to prove it was related to drugs without your admission, which would be hard to do.
If you take the 5th your useless to them and you protect yourself. There is no prof you personally made the phone call or relieved it, for all you know santa clause could have made that call. You said they claimed they would get a warrant, if it is in relation to this case it would most likely be an obstruction of justice
warrant. This is because if your not incriminating you self and you appear to be a material witness and you are hindering their investigation
, it could constitute obstruction of justice, but your 5th amendment right against self incrimination trumps that. If there are other matters for which they could get a warrant for, then the situation is a bit more difficult as then they have leverage. If they use that leverage and you are worried, make a statement, but as of that point you know of no other drug besides pot, and you have never seen anything larger than 40 grams. If you get immunity from any violation concerning controlled substances, from the state attorney, it is important you truthfully answer because a lie at that point may nullify the agreement and they might be able to use your word against you. I still don't think they are worried about your drug use a year ago, but i cant say for sure, it's hard to say over the computer
without talking to the police or knowing more about the incident.