The Short Message Service Centre (SMSC) is a service provided by your mobile phone operator. The service it provides is receiving, storing, routing, and forwarding SMS messages. It basically acts like a courier service for your SMS messages. Your mobile phone sends messages to the SMSC, and the SMSC handles the rest. If the recipient is on the same network and their phone is on, then it sends it direct. If the recipient is on a different network, then it forwards it to that network's SMSC. If the recipient's phone is not online, then it keeps trying to send the message for around 3 days.
So it's really important to make sure that the SMSC number is correct, otherwise you may not receive or send messages. And in fact, even if you can receive and send messages in your home country, it could mean that you won't receive messages when you start roaming to another country! This can be a really bad thing if you can't receive your two factor codes say for your credit card (happened to me!!)
To update the SMSC number on your Android phone, follow these simple steps:
Press the power button on your phone to lock your phone.
Press the power button on your phone to unlock your phone.
On your lock screen, in the top left corner, it will display your mobile carrier name.
In Google, type in the mobile carrier name and SMSC (e.g. telstra smsc).
Check the Google results for your mobile carrier's SMSC.
Make a note of your mobile carrier's SMSC. You'll need this later.
Open the dial pad on your phone.
Dial *#*#4636#*#*. A secret menu will appear!
Select “Phone Information” from the options displayed.
Depending on the SIM card slot you want to update (SIM 1 or SIM 2), select either “Phone 0” or “Phone 1.”
Scroll to the very bottom of the page.
Look for the “SMSC Setting” option and select it.
Hit "REFRESH".
Copy down this phone number as a backup.
Replace this phone number with the SMSC number that you found from step 6. You'll need to include double quote marks like this: "+61469000111",145
Tap on “Update” to apply the changes.
Restart your Android phone by holding down the power button and choosing RESTART.
There's another set of important settings which is your Access Point Name (APN) and IP Multimedia Subsystem (IMS)
The APN provides the gateway settings your phone uses to reach the mobile data network. It includes the APN name, username/password if needed, and optional fields for MMS. You can think of it like your wifi access point name + username + password except it's for mobile data.
The IMS are the details for modern voice and messaging over 4G/5G, such as VoLTE, SMS over IMS and Wi‑Fi Calling.
Google used to work with carriers to provision this automatically using the Carrier Services application. But it's not clear any more if Carrier Services updates the carrierconfig.xml file on your phone. I mean it used to, but now it seems like Carrier Services only does RCS. May be Google does this through Google Play updates, or perhaps just relies on your mobile carrier to have the correct APN/IMS (which it probably doesn't if your mobile carrier is an MVNO or you're using an eSIM).
Anyway, it means for Android phones, you'll likely have to update the APN and IMS details manually. This is especially important if you are downloading an eSIM when travelling! Make sure you check this before you arrive or you'll have to do the debugging when you land (like I had to with Saily).
Here are the instructions on how to check and update your APN and IMS:
Go to https://www.apnsettings.org/ and look for your mobile carrier. Alternatively you can google your APN settings for your mobile carrier or contact their tech support.
Go to your home screen by pressing the home button to show all your apps.
Open the "Settings" app.
Network & internet > SIMs > pick your SIM > Access Point Names
Check the APN settings against what you found in Step 1.
If it's the same, then you're good! Your APN and IMS details are correct.
If it's different, then in the top right, click the "+" sign.
Enter the details from Step 1.
In the top right, click the three vertical dots and choose "Save".
Click the circle next to the new APN.
For more details see https://ola.tech/en/blog/android-iphone-apn%E8%A8%AD%E5%AE%9A/
One final thing, it looks like Google Pixel 6 and above may be region-locked, such that editing IMS settings doesn't necessarily mean that the phone will work.