Outbound Proxy Server When you look at the configuration options on most IP phones, you will see a field called "Outbound Proxy" or "Outbound Proxy Server". This is an important part of the configuration because it determines how outbound calls are routed. When the IP phone makes an outbound call, it sends the request to the outbound proxy server. The outbound proxy server examines the request and looks up information in its internal databases to see if the client that is making the request is allowed to call that number. If the call is to a PSTN number or other chargeable destination, then the user's credit level may be checked before allowing the call to proceed. If the call is permitted, the proxy server will then check the caller's identity to make sure it is not a rogue client trying to impersonate a valid user. This is done by sending a challenge back to the calling device requesting identification by way of user id and password. The contents of the SIP messages that carry this identity information are encrypted so it is not possible for others to intercept the message and read their credentials. If the client is able to authenticate successfully then the call may proceed. Part of the outbound proxy's responsibility is to decide where it should forward the request next. If the outbound proxy gives access to the PSTN then it might need to forward the call to a PSTN gateway. However, if the call is to another SIP service, perhaps one operated by a different VoIP service provider, then it will look up the correct destination using DNS. Setting for use with a commercial VoIP Provider: If you want to use an IP phone with a commercial VoIP Service Provider on the Internet, such as sipgate, telappliant or callcentric, then the provider will tell you what to set as the outbound proxy server. It will be the name or IP address of one of their proxy servers. Setting for use with an IPBX: If you want to use an IP phone with an IP capable PBX, then the outbound proxy server will need to be the PBX. This might be in the form of a host name or IP address, but you will need to find out this information from the administrator of the system. Setting for use with Asterisk or other "home grown" solutions: The outbound proxy server would normally be the Asterisk server. The Asterisk server would then need to be configured with a dial plan that handles outbound calls from the clients. This dial plan might implement rules that prohibit callers from dialling certain numbers (e.g. premium rate numbers) or that routes calls out onto different trunk connections depending on the dialled number (a form of "least cost routing"). The dial plan would also need to recognise when the dialled number matches the pattern for another internal phone on the Asterisk PBX system. It is important to choose a numbering scheme for internal phones that can be easily distinguished from PSTN and other external numbers. It is also useful if the numbering scheme selected for internal IP phones is different to that used for internal analogue phones where both must co-exist. Do I have to register with the outbound proxy server? The simple answer to this is no. You should be able to use an outbound proxy server without necessarily registering first. The registration process is crucial for your IP phone to be able to receive inbound calls, but it will not always be necessary in order to make outbound calls. If the call you are trying to make would incur call toll charges then you will almost certainly need to have a valid user account with the provider whose server you are trying to use (unless there is some arrangement in place for cross charging between providers). However, that does not necessarily mean your phone has to already be registered before the call starts. Instead, the authentication of your phone will occur as part of the SIP dialogue that occurs when the call is initiated. Another fact that may be relevant to this question is that the registrar server and the outbound proxy server may not be the same. In order to spread the load on busy systems, registrations may be handled on a different server. My IP phone allows me to specify more than one Outbound Proxy Server. Why? Some IP phones will allow you to enter details for more than one SIP Proxy server and then to nominate one of them as the default. This is a useful facility because it means you can nominate a specific Proxy server for certain numbers in the address book, while ensuring there is a default route for all other numbers you may want to call. Unless your IP phone is being used only to make internal calls on the company phone system, the default outbound proxy server would normally be the one that gives you access to the PSTN. Must I have an account with the provider before I can use their Proxy Server? It is not always necessary to have an account with the service provider when you want to use their server as an outbound proxy. For example, if you wanted to call an IP phone that you know is registered with sipgate, even though your phone is not registered with sipgate, you may be able to specify a sipgate server as the outbound proxy for that call. This is because the call will not incur any toll charges and would have to eventually be routed to one of their servers anyway - in effect you are cutting out one or more intermediate proxy servers by specifying the correct final destination proxy server in the first place.