If you connect two (or more) VOIP phones or ATAs all set up with same Log In details to the same router or switch, what happens? Do they all ring and you can answer any phone?
If there are several handsets/end-point devices behind your router registering to the same SIP host in the cloud, then the best practise advice advice is to not use the same SIP port numbers on each end user device, but to increment by one so that they are different. e.g. see advice below from Sipgate
This is subtly but fundamentally different to having one device (e.g. an asterisk box - a software PBX basically) behind your router handling multiple SIP registrations to the outside world and then having multiple handsets that connect to it, rather than a SIP provider directly to multiple handsets. If that makes sense.