It would be a resonable certainty if the letter found at the Ponderosa restaurant is from the sniper. The call was reputedly made to a phone number that was specified in that letter. This number would only likely be known to the author of that letter. It would be a fair assumption that other info contained in this letter was also referenced in the conversation. Again info only the author would likely be privy to.
If the reports now circulating prove to be accurate, that a second and similar letter was found at the scene of yesterday's shooting, then the likelihood of the letters being from the sniper increase to near certainty as well.
The possibility of there being more than one sniper, or that the sniper has some accomplice(s) could mean that the caller is not actually the trigger man, but the difference in terms of the investigation is fairly insubstantial in that case.