Ringing group don't work

hello all,

i have read here and there how to setup a ringing group, to make a call reach more than one phone at the same time, then just only one answer.

So i have the end of my exstensions.conf as below:

[my-ring-group]
exten => 500,1,Dial(SIP/jhome&SIP/office&SIP/sp)

home office and sp are users created in sip.conf as:

[home]
type=friend
username=home
secret=xxx
nat=yes
host=dynamic

The problem: when i call the 500 extension, the pbx pick up the call, immediately, but the extensions are not ringing anyway. I don’t see any network packet routed to the extensions, the pbx just answer the call to the caller.

If any help,

greetings
angelo