[Asterisk-Users] Call waiting does not ring phone

[Wilson Pickett]

Can someone please tell me if this is how call waiting should work?

ZAP/1 and ZAP/2 are X100P on phone lines
ZAP/3,4,5 have phones conected.

If a call is in progress between ZAP/2 and ZAP/3 and 
ZAP/2 gets a callwaiting signal which ZAP/3 hears

the Dial command has SIP/2002&ZAP/3&ZAP/4&ZAP/5

If ZAP/4 is off hook, I hear the call waiting signal

The CLI says it is ring the 4 channels above

But the ZAP/4 phone, when on hook does not ring.

Can someone tell me what settings might cause this?