How to organize VoIP/SIP in a small office?

Question

SIP

Greetings.
Gentlemen, I was faced with the task of organizing a telephone network in the office, although I had never dealt with this before. I read the material on the toaster, habré, sat in Google, but still there are questions. Advise-help, please.
There is a Moscow office with wired internet. It is required to organize 2 telephone numbers for 3-4 people (the first number is personal for one person, the second for the remaining three), from which it will be possible to call within Russia and very rarely abroad, to which everyone can call (ideally with the ability to use an extension, to get to the right person) and, of course, communication between all users using internal numbers.
As far as I understand, you need to buy a number and create an account with a SIP provider; purchase a piece of hardware that performs the functions of a VoIP gateway and configure it to the appropriate SIP provider account; connect a PBX to a piece of iron (and set up internal phones) or a SIP phone with additional handsets. If I didn’t mess up anything, then the circuit is quite simple, but there are questions about the details.
1. Is a VoIP gateway required (some LinkSYS SPA3102) or is some ZyXEL LTE (or any other router) with FXS ports sufficient? Advise, please, decent pieces of iron, who understands.
2. Can any analog telephone and/or PBX be connected to the FXS port? The operation of such a port is no different from the "telephone socket in the wall"?
3. Does the SIP phone with additional handsets have a built-in VoIP gateway and is connected to the router's Ethernet port?
4. Is it possible to remove the PBX in such a scheme and uncommutate internal calls through the cloud PBX of the SIP provider, through the settings of the VoIP gateway, or through the settings of the IP phone?
5. If the provider supports fax transmission, will it work with any connection option?

Answer 1

Now every first provider offers its virtual PBXs with external / internal numbers and installation out of the box. And no dancing with Asterisks, settings, and so on. Convenient :)
In Moscow, for example, we use telfin.

Answer 2

You buy sip-phones (they connect directly to the local network), buy numbers and a "virtual PBX", for example, from mango-telecom or for free.
You register each handset through the web face in the PBX.
Option number 2
Buy sip-phones and sip-numbers from the provider. Set Asteriks (You'll have to tinker with the settings) or analogues (software PBX)
Option No. 3 (If you plan a large company growth)
Buy phones and an iron PBX (For example, Panasonic KX-NCP 500 / 1000). You connect with the provider, register handsets on it, prescribe routing, configure DISA, etc.
Option number 4
Contact a special organization that will calculate everything, bring it and set it up. Then you don't need to sweat at all.
If you need exactly analog phones:
Option No. 1 - Any voice gateway with SIP support
is purchased Option No. 2 - A fee is purchased in the computer where you can connect telephone lines
Option No. 3 - Depends on the PBX (The fee is either included or purchased in addition)
Option No. 4 - They themselves know what to buy, the main thing for you is that everything works.

Answer 3

https://habrahabr.ru/company/Voximplant/blog/212713/
And don't overpay :)