What happens when two SIP clients log in at the same time

Question

If two SIP clients are logged in to a service provider at the same, what happens when an inbound call comes and what happens if you try to make an outbound call? (This question is regarding SIP service connected to a US phone number.)

Do any of the following affect the answer to the above question?

• Using a different SIP service provider
• Using different classes of SIP clients, e.g. my desk phone and a command-line client that just collects live caller ID information

Thank you!

Answer 1

The ultimate answer depends quite a bit on the service provider's configuration.

Option 1: ring all registered endpoints (parallel branch), wait for any one of them to answer, then cancel the remaining requests.

Option 2: ring only the most recently registered endpoint.

Option 3: ring each registered endpoint one at a time (serial branch) until one answers.

The fact that multiple registrations exist isn't as significant from an outbound calling perspective. Registration is primarily so that the provider's proxy knows where to send inbound calls. When you call outbound, you're typically also reauthenticating. Whether the outbound call succeeds depends more on configured trunk capacity on the provider's end.

Some providers offer a fixed number of active inbound/outbound calls and you won't be able to exceed that regardless of the number of endpoints. That being said, up to that limit, it's typically fine to make any combination of inbound or outbound calls from any of your endpoints (even multiple from the same endpoint).

As far as the types of endpoints/clients you're using, most of the decisions about what to do to route an inbound call happens at the proxy. Your endpoint will either receive the call or it won't and it can decide to answer or not (or redirect/transfer to voicemail).

Answer 2

The chances of this happening at the same instant, to me, seems very rare. Servers, in any case, do not participate in the calls. Once the two endpoints have negotiated a successful call set up, the server back out and is ready to process the next call request. The server, in reality, send the call request to an SIP proxy that connects the two end points and decides on the SIP packets that are exchanged between the two.

If by a freak chance, there are two calls that clash at the same instant, I suppose the server will prioritise one and, maybe, drop the other. I am also quite confident that modern hardware and software will work to form a queue and handle both one after the other.

For those trying to understand SIP trunking, there is a simple to understand infographic at http://telecom.toshiba.com/multimedia/content/sip-trunking-101.cfm. Share it with your friends.

Thanks

Answer 3

You may want to take a look at this, if you are trying to implement something yourself: https://www.google.com/patents/US20120157098