Oops. My cut and paste mistake from another conference bridge!
The IAX2 connection syntax should read:
exten => 8502,1,Set(CALLERID(all)=YourCall<6785551234>)
exten => 8502,n,Dial(IAX2/callin@callin.xelatec.com/8502)
If you need a hosted Asterisk solution, Aretta Comms is a great choice!
Steve, W9SH