[asterisk-users] registration limit

[Nhadie]

Hi,

Is there a way to limit only one registration for each user at a time?
meaning if a user tries to register, but that user is already 
registered. i will deny?

or is it possible to for  a single user at the same time, and when 
someone calls that user, it will ring both phones?

Just want something whereby a user can assign his extension on an IP 
phone in the office, and assign the same thing maybe to a softphone on 
his laptop or maybe a sip client on a mobile phone. so that whenever he 
leaves the office he can still be reach on his extension via the 
sotphone. thank you.

regards,
Nhadie