Adding effect ON an effect..

Instead of putting the delay on the track, create a send bus.  Send as much delay as you need to the bus via the send level with bus inline on the bus effect bin.  Route the bus to the master (or the vocal bus if using that and you want your effects w/ the vocal bus effects).  Get a nice level like what you want between the dry vocal and delayed version.  Then start EQing the delay bus to get the telephone effect.  After that sounds good, you'll probably have to raise the volume of the bus send again.  Rinse, repeat until you have the right delay sound and it is loud enough compared to the dry vocal.
 
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