[asterisk-users] Handle a call if one phone of a ring group is busy

[Frank]

Hi List

I have a phone in my living room (ext. 111), a phone in the kitchen
(ext. 222) and a phone in my bedroom (ext. 333).

Both phones are part of a ring group.

exten => 7654321,1,Dial(SIP/111&SIP/222&SIP/333)

Everything work fine and, as expected, all phones are ringing by an
incoming call and I can answer the call on the nearest phone.

Problem: if there is a second incoming call while I'm talking on one of
the three phones, the other phones ring and I cannot answer the second
call since I cannot be at two places at the same time. :-)

Question: How to give a "busy signal" back to the caller if one
extension of a ring group is in use? Or redirect the call to voice mail?

Any hint?

Thanks in advance 

Frank