Inter-domain calling

Also known as "federation", calling between domains is a default feature of the Kamailio server. If you are registered as (on server and your friend is registered as (on server, simply call to ring Bob's phone. Another option under the federation umbrella is commonly understood as "call forwarding". For example this will allow users with custom domains to add information to their own domain to allow calls to to ring a client registered to . This process is more complex and covered in the Server Documentation.

Secure VoIP interoperability table

Service URL federation secure yes yes
Linphone yes yes
SilentCircle no yes
Crypto5 no yes
RedPhone no yes

The larger question of true secure voice federation is much more complicated than giving the user the ability to make calls between domains. The most common question I'm asked about is "can I make secure calls to a cell phone or landline?" The answer is always a reluctant "no". The reasons are much more verbose. I'm going to use the acronym PSTN for "cell phone or landline" from this point forward.

  1. PSTN providers don't care about your privacy
  2. CALEA AKA the USA wiretap law. Other nations have similar wiretap laws for PSTN calls
  3. The PSTN is a 100+ year old network
  4. Phones provisioned for the PSTN do now allow user control of the device
  5. PSTN providers disrespect network neutrality, thus do not allow custom security protocols to run on their networks

To summarize, secure calls from to the PSTN are theoretically possible, pending the event whereby PSTN providers allow secure protocols to operate on their network. I do not predict this event is likely.

The larger question of secure VoIP federation within independent VoIP providers is also more complicated. For example, there are many providers that have built a commercial service using the open source stack. For these companies, federation is in direct conflict with their business plan. They prefer a walled garden whereby both calling parties must subscribe to the company's service to communicate securely. While this is a tempting opportunity to grow capital, it harms the overall network by isolating users into silos. These companies have the technology to federate, though they choose to forbid federation!

For example, I've contacted the company Crypto5 (which uses the open source ostel stack for their backend service) about federation between their service and I was informed that they have no intention to allow any of their users the ability to place secure calls outside of the domain. The admin was surprised I asked him that question in the first place! It seemed obvious to him that a commercial service would want to silo users to forbid unrestricted secure communication.

Another example is the popular service Silent Circle. Their Silent Phone application uses the SIP protocol, which is the same protocol as, though they do not allow calls from outside the domain. They also offer the Out-Circle service which advertises secure calls to the PSTN! This is misleading, as calls are only secure from the device that subscribes to Silent Circle! The cell phone or land line on the other end is insecure! This fact is not mentioned anywhere in their public information.

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