A SIP Broker "alias", is a SIP Broker "number" that starts with *011 (as dialed from SIP Broker) that is translated (by SIP Broker) into the full SIP URI (internet phone address) to reach your VoIP adapter. Essentially it is a "number" that is dialable (using JUST the normal keys on a phone) that can reach you by anyone using SIP Broker. You have to already have a valid internet VoIP address that can ring your adapter, but the SIP Broker "alias" can make it easier for SIP Broker users (including users of the free SIP Broker PSTN inbound numbers) to easily dial your adapter. Essentially it just gives you an easy "alias" for your existing VoIP adapter address.
NOTE: In addition to making it "easy to dial you" with a normal phone (i.e. translate out all the non-numeric characters), this can also be useful for those using dynamic DNS addresses that only have DNS "A" records. Because SIP Broker understands both "A" and "SRV" records, it can create a working "alias" for a VoIP adapter listed by type of DNS entry. And because the sipbroker.com proxy is listed in DNS both by "A" and "SRV" records, it can be reached by VoIP equipment using either type of DNS. This means that a person who has only a (dynamic DNS service) "A" record for their VoIP adapter, can create a SIP Broker alias for that ("A" record only) SIP URI. But if they tell others that their SIP URI is *011aaa@sipbroker.com (where aaa is replaced with their SIP Broker alias, including "Country code"), then that SIP URI they give out will be both usable by services using "A" DNS and VoIP adapters and services that only use "SRV" records (as the SIP Broker "alias" support SRV records, even if/when your SIP URI is "A" DNS only)!
SIP Broker aliases are free, but (unlike most SIP Broker features) they do require you to sign up for a free SIP Broker web account. You can sign up for a free SIP Broker account from the main SIP Broker page: http://www.sipbroker.com/sipbroker/action/login . When you do so, be sure to pick as your "userid" the VoIP address that reaches your adapter. For example, if you use "Free World Dialup", and have 123456 as your "number", than your userid should be 123456 at fwd.pulver.com (replace this example, with whatever SIP address would reach your adapter). The password can be anything you want, but the userid MUST match your SIP address, in order for the "alias" feature to work with SIP Broker!!!
Now that you have a SIP Broker account that matches your SIP URI, you are 1/2 way there to having your own "alias". The only other part of the setup, is to fill in the "Country" and "alias" fields under you account "Member Details", and save the changes. The "Country" is just used to pick the dialing "Country code" that comes before the alias.
For example, if your home phone number was 1-555-555-1234, and you wanted a matching SIP Broker alias (that would actually be dialed via SIP Broker as *01115555551234 ), you would enter in "United States" for the "Country" (because the USA is international dialing prefix 1), and 5555551234 as your "alias". Voila, people should now be able to reach you with SIP Broker dialing by using the alias code prefix *011, the country code 1 (USA), and 5555551234 as the actual number (i.e. dial *011 1 555 555 1234 from SIP Broker, including the free SIP Broker PSTN numbers). Whenever SIP Broker sees someone trying to dial your "alias", it will auto-convert the call into a call to the SIP URI matching your SIP Broker Userid.
At that point, you might want to try testing your SIP Broker alias. One easy way to do this, is to use a normal phone to call one of the SIP Broker PSTN numbers http://www.sipbroker.com/sipbroker/action/pstnNumbers from some other phone (a cell phone, perhaps?), and then try calling your "alias" at the voice prompt (remember all SIP Broker alias dialing starts with *011, followed by the "country code" you listed, followed by what you have as your "alias"). If your VoIP adapter "rings" and you can talk, your "alias" is now working, and people can easily use SIP Broker to dial your adapter "for free" -