Answering a second call from the queue

Hi There,
Hope you can help us. We are using asterisk with about seven single extension sip phones.

we have a ring group which rings every phone at the moment when an incoing call is detected.

Our problem is that if you are already on a phone call and a new call comes in you cannot answer the new incoming call. You have to move to another phone to answer the call.

The phones reregister themselves after every call. The system has been running for some time and works just fine apart from this annoying issue.

Is there a way of saypunching a code into the phone to answer the next available incoming call? or is there a way to configure the asterisk to allow the phone to answer the new incoming call.

Any help is appreciated.