you have three settings in the RDR data block in LD 15. To print do this in LD 21.
Req: prt
type: rdr
cust: 0
enter, enter..yada..yada..
Look for cfn0/cfn1/cfn2...those are number of rings.
You can change them to whatever you need.
then on the phone look for RCO (Ring Choice Option? or something like that) and use 0, 1 or 2 to match the CFN settings.
You have to be sure you don't change one someone is already using, and sorry, yes, you only get three options.