Background

Note: As a user, you will not need to read this page, in order to use the IFI VoIP system. (So you may stop now ;) ) However, it contains some some background information, which is useful, if you're trying to debug your setup/configuration, if something doesn't work as expected.

SIP phone addresses consist of a username and a hostname part (as do emails).
Your username is the 5-digit extension you were assigned initially (e.g., 50890).
The hostname of the IFI VoIP server depends on the interface, you're connecting to:

• internal (w.r.t. the UZH firewall) : voip-internal.ifi.uzh.ch
• external (w.r.t. the UZH firewall) : voip-external.ifi.uzh.ch

These are resolved to the proper IP addresses using DNS (Domain Name Service), like any other domain names, you use.
Thus, your real SIP VoIP address looks like this: 50890@voip-internal.ifi.uzh.ch

Since this address looks somewhat ugly, there exist so-called SIP SRV entries for the ifi domain. These are comparable to MX entries used for email communication, and map
ifi.uzh.ch to voip-internal.ifi.uzh.ch
Therefore, Users that use a SIP SRV compatible VoIP client, may reach you under an address, like: 50890@ifi.uzh.ch

For convenience reasons, an alias exists, mapping your IFI username to your VoIP username/extension, allowing users to call addresses, like: schaffrath@ifi.uzh.ch

In order to be compatible with the old telephony system, where all addresses consist of digits only, the ENUM entries were created. These are responsible to map traditional phone numbers to SIP addresses.
In order to achieve this, DNS was again (ab- ;) ) used, creating so-called NAPTR entries in specific top level domains.
The IFI VoIP system is registered in the nrenum.net domain (responsible for swiss research institutions).
Thereby, 0.9.8.0.5.3.6.4.4.1.4.nrenum.net resolves to 50890@ifi.uzh.ch .
Thus, callers, which are registered to a VoIP provider that performs ENUM lookups, may call numbers, like : 0041446350890
(NB: Some providers will not perform the ENUM lookup for all adresses and establish a connection via their PSTN gateway nontheless - and thereby earning money with the call ;) )

The ENUM setting in your user settings does not change anything about your reachability, but only the setting in the IFI VoIP system, whether an ENUM lookup will be performed, when you're calling someone else.

So the resolving looks like this:
0041446350890 ---ENUM--> 50890@ifi.uzh.ch --SIP-SRV--> 50890@voip-internal.ifi.uzh.ch

IAX2 is merely another VoIP protocol.
It was originally intended for communication between VoIP servers, but nowadays, some VoIP clients support the protocol as well.
It's got some advantages over SIP w.r.t. firewalls, as it uses only a single symmetric (i.e. with source-port == destination-port) UDP connection for both signalling and actual voice data. SIP is only used for signalling, the actual data is transferred over RTP (Real Time Protocol) on dynamic ports, thereby sometimes creating problems with firewalls, if the clients are not correctly set up, to use symmetric ports as well)

IAX2 addresses look like caller-username@hostname/callee-username .
In order to allow users, who are not registered to the destination host, to call IAX2 extensions on that host, a guest account is in place.
Thus an IAX2 address for external callers would look, like: guest@voip-internal.ifi.uzh.ch/50890

Also here, ENUM, alias and IAX SRV concepts are applicable, as described above.