The clicking is the system trying to find the phone. Every carrier has there own method, Nextel uses "Please hold while the Nextel subscriber you are trying to reach is located" and Verizon just inserts fake rings, but Alltel uses that clicking noise to let you know the phone is being located...

Now Im learning more and more. I have had nextel in the past and that lady telling me to please hold while she locates the nextel subscriber used to bug me out. I hate her but now I understand the whole alltel clicking thing and nextel thing. Thanks ofr the heads up 😁