Today the cfb/cfu/cf is using the ringtimer from the called extionsion and that creates some issues. For example - call the extension for 15 seconds and then cf to a ringgroup - the ringroup will only be ringing for 15 seconds before returning to the called users voicemail. Sometimes you don't want to have this behaivior, you want it to ring on the ringgroup "forever".
The patch will come shortly will introduce a second ringtimer (per users) and with 0 as default - this will copy the present behaivour. Any postive number would be used in cfu/cfb/cf. A value of -1 would instead use a Goto(from-internal,cfdest,1) to skip the localchannel.
In the case of cf there are som things to look out for as phillipe pointed out in #4332