There is no SIP method that will force a phone to answer.
There are additional SIP headers that will cause some SIP phones to answer an incoming call immediately. These may vary from brand to brand, and because they represent a security vulnerability, the phone should not honour them unless the feature has been enable in the phone.
You can pick up an incoming call on a phone that was not the original target for the call and which has been manually answered, by using ChannelRedirect.