Filing a third-party claim is much like filing a claim with your own insurer. However, you should contact your provider and tell them about the accident first, even if you’re not at-fault.
This way, Farmers can help contact Allstate for you. (You’re also contractually required to contact your insurer after any incident with your car). If you still want to call Allstate yourself, you can: just give them the other party’s name and policy number, if you have it.