In this post, we’re going to be looking at the question, how do phone calls work? I mean all we do is dial a number and someone picks up on the other end. Magic? Right? Wrong.
There’s so much going on in the background that we do not know about. This post intends to lay bare all of that. Firstly we’ll look at
- IMSI
- Mobile Switching centers
- How do phone calls work?
So in the last post we looked at how mobile cellular network connections work and explained it using a simple call.
The summary of that talk was very simple. Your phone is always connected to a cell tower wirelessly. That’s how you get a signal. When you make a call, the signal from your phone goes up to your connected cell tower.
The cell tower then passes that signal to the cell tower that your call recipient is connected to. It does this either via wires (optic fiber cables) or wireless via microwaves.
The recipient’s cell tower then picks up the signal and broadcasts it to your recipient’s phone.
This would then lead us to the question, if you want to call a friend, how do the cell towers know where to find the said friend?
Before we start to answer this, let’s look at what IMSI and MSCs are.
International Mobile Subscriber Identity (IMSI)
When you buy a sim card, you’re purchasing a cellular identity card. Every registered SIM card has what is known as an IMSI or international mobile subscriber identity.
This IMSI carries a series of unique digits that can be used to identify you anywhere in the world (even if you’re roaming).
The IMSI can also be used to access other details that you submitted when registering the said SIM card. These details include your name, age, sex, photograph, address, etc.
Mobile Switching Center (MSC)
This information is saved to what is called an MSC or a Mobile Switching Center.
The Mobile Switching Center or MSC is a building facility that is usually located in the core or central point of a cellular network.
When you buy a SIM card, all of your personal details and IMSI are saved to the MSC that is closest to the home address that you gave.
This MSC would henceforth be known as your home MSC.
This information would then be shared with all other MSCs that are controlled by network providers. This is so that all those other MSCs know where to find you.
Your home MSC stores your data/voice plans, present location triangulated by at least 1 cell tower or by 3 cell towers (optimum) and activity (online or offline).
When an IMSI is combined with an IMEI ( a unique ID number assigned to all smartphones), it becomes a very effective tracking tool.
I digress.
If by any chance you leave the geographical location of your home MSC and go to a new location, you’ll be assigned a new temporary MSC (or Foreign MSC).
These Foreign MSCs will then keep your home MSC updated about your location and activity. That way, your home MSC will always know where to find you inorder to relay calls to you.
Your home MSC usually updates itself about your location:
1. Time based (every 3 to 6 hours depending on the network provider)
2. Using data from the foreign MSC
3. Whenever the phone comes on
This is why when I used travel to Sokoto, after a day or two, I would start receiving promotional messages in Hausa.
This is because my network provider knew my general location.
How do phone calls work
Now let’s explain how this works, again using a very simple phone call.
Let’s say Sarah wants to call Michael…
Sarah dials his number from her phone and her call signal goes up to the nearest call tower in her location.
The cell tower sends light pulse signals via optic fiber cables or wireless microwaves to Michael’s home MSC.
Michael’s home MSC would receive the signal and check for his current location. If he’s at home, the call would be forwarded to cell towers in his current location and his phone would ring.
If he is not at home, then the MSC would forward the call to the foreign MSC where Michael is located.
The foreign MSC would accept the signal and then send it to cell towers close to Michael’s last known location and all things being equal, Michael’s phone should ring.
And that ladies and gentlemen is how your phone calls get from point A to point B.
In conclusion
So long as your phone is on and operational, you cannot hide anywhere on this planet Earth that has network reception.
The cell towers are eyes in the sky that watch over you lol.
If your network provider is served official court documents demanding your data and the location of all the places you have ever been for the last 10 years.
It can easily be collected from your MSC.
Even when you think your phone is off or on airplane mode, so long as battery is still inside, it will keep talking to cell towers no matter where you are.
Edward Snowden alleges that this is one of the reasons why smartphone manufacturers made batteries non removable. A claim that smartphone manufacturers like Apple have denied stating that they care about users privacy and all that.
With that we’ve come to the end of this post. Please check out other posts on the site. There’s tons of posts that you’ll love.
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