You can register multiple end users with only one SIP account but Asterisk does not support ringing all the registered phones on single account. Whenever a new registration comes, asterisk updates its contact info in memory. So if the registration is coming from multiple end users (multiple ip address and port) then the call will be placed to the phone who sent latest registration request. Asterisk does not keep track of all the IP Addresses for single account registration.
What we have done to ring all the end users with same account is that we listen to registration requests thru Manager API in order to detect multiple registration. If we have detected multiple registration then we store the contact information of all the end user phones which are related to single account. And when asterisk receives a dial request for that user, we create
a temporary/fake users (as many as needed) in memory and dial all of them in the code not thru Dial application as it does not support this scenario.
We are still working on this scenario. It is in working condition but in testing phase.