Well over a year later, there is a follow up to an earlier focus on Session Initiation Protocol and can it work just having some SIP phones tied together without the expense of time and resources of having a phone system?
In short, if your router is calling you, that call may be a hopeless outcome.
Basically SIP is the modern-day landline with an IP stack, with a functionality similar to the web. You use Safari or Chrome to go onto say Facebook or The Museum. In the SIP context, the phone is the browser, and the telephone number is the Web address.
Search results in the past say you can use SIP without any proxy, gateway, etc. by zero-ing out the said addresses. Therefore these devices can work by dialing the latter portion of the IP address, so if your IP address are in the 192.168.1.0 range, you could call an IP address of one phone of 192.168.1.13 by dialing 13. This is called “Direct IP Calling”, and should remain for this single use case.
If you have some knowledge on this subject, you may ask, well “what’s wrong with that? Isn’t this part of VoIP?” If you want to have SIP be part of the greater cyber world, it has to be done in the form of Uniform Resource Locators or URLs much like melanie at clickford dot net. or steven dot clickford at clickford dot private. SIP requires a bunch of other Internet protocols to be set up simultaneously. SIP was to allegedly be more of an “open standard” to the already open standard of H323, that was a bit more opaque but SIP is just as guilty.
What does this all mean? Some of the major vendors wanted to put some additional features to work more like an office telephone, since SIP is basically a landline with all the standard 19 features you can currently get from your broadband provider, and if you’re a 90s kid, you had the Baby Bell service when Call Waiting alone was $3 x/month. Essentially to have PBX-like features, the vendors had to get creative and make SIP an open standard to troubleshoot and monitor the quality by keeping the logs easy to read. A lot of the forwarding, ACD, and Camp On like features would be done discretely through making a button called Features, that basically opens up to what appears to be web page with scripts to go back to the phone system to activate some feature.
By theory, Mitel, Avaya (Nortel and legacy Lucent), Cisco can operate phones off their native PBX or Key system, but it’s again it becomes a landline with an IP stack. They can work like a home phone, with a couple more “appearances” that acts like a visual call-waiting, and be able to do basic telephone calls.
Another issue is “ghost calls”. This is often referred to be malicious and cause toll fraud, but for isolated, LAN based SIP telephony, this means if a rogue telephone that doesn’t comply to SIP standards, a phone will ring off the hook for hours, maybe days only because the ringing telephone didn’t get a proper SIP signal or because you are calling it by an IP address, or part of one, this can also cause issues.
In the age of wireless, and the erosion of landlines, SIP has matured to be basically a Landline telephone service to modernize that dial tone for sites with office telephones. It’s great for trunking, in fact the only Internet telephony standard that supports Caller ID systematically. When it comes to phones, if you expect it to do a lot of what you have expected in legacy Nortel, Avaya or heck even Cisco, it’s just as functional as a phone you would’ve bought at Radio Shack, if only they were still around to sell disposable IP phones. But this “open” protocol has it’s downsides, it’s open to 1,001 different issues, quirks and if one phone has an older firmware and has a bug and another phone of the same type with newer ones, this would cause even more headaches, as other centralized VOIP systems, the bugs live in the router, brain or server, and not on the end device.