Are phone pranks illegal?

Yes. If you cause another phone to ring/signal just to annoy the owner and if you allow someone to do that with a phone which is yours, you can be found guilty of a Class 3 misdemeanor. A second or subsequent offense becomes a Class 2 misdemeanor. Records of calls made between one number and another can be obtained without your knowledge or permission by the prosecution. 

Further, if you annoy, harass, hinder, or delay emergency personnel in the performance of their duties (public/private entities providing fire, police, or emergency medical service) or permit the use of a phone under your control, you may be charged with a Class 1 misdemeanor.