You can setup multiple sub-accounts, each with its own extension (101, 102, 103,104....) and have multiple registration to the same account. You can also call between sub-accounts by simply dialing the extension number. There are other neat tricks you can do with sub-accounts but they escape me at the moment. So, you can have one sub-account register from your office ATA (101), another from your home (102), another from your mom's house (103) and another in another Country (104). If you want to call your mom from your office ATA, you simply dial 103. These calls are free!
Edit: You can also setup ring groups with a group of sub-accounts (eg., 101, 102,103) and I think, a form of failover, in the event one of the sub-account is not reachable, etc.