First restart your mobile phone. Make sure of the following:
SIM card has been successfully unlocked
You are in a location with good mobile phone reception.
Re-register the SIM card with the network
If restarting the mobile phone is not enough, the card must be re-registered in the network. This can be done via the mobile phone, but may also require the SIM card to be removed. The operating instructions of the mobile phone should contain information on this.
Check whether the mobile phone is set up for SMS reception.
Enter the code *222# via the keypad of the phone app and confirm with the Call key. With many providers, this activates or re-activates SMS reception.
Check the SMS settings in the smartphone
Settings > "Network & Internet" > "Mobile network" > "Advanced". The menu items may differ from manufacturer to manufacturer, but are usually similar.
For example, if WLAN telephony is activated here, SMS messages are sent via the WLAN network. If there is no WLAN reception, SMS will not work either.
Deactivate the option and check whether the SMS then arrives or is sent.
Please make sure that SMS forwarding is not activated!
Check the number of the message centre
You can ask your mobile phone provider for this number. The number of the message centre can usually be set via the options menu in the message application of your mobile device. These numbers may vary depending on the network provider and/or reseller.
Example for Samsung devices:
Open the Messages app.
Tap the three dots ⋮ at the top right and then "Settings".
Tap on "Advanced settings". If the menu item does not exist, skip to the next step.
Tap on "Message centre".
Enter (or check) the number of the message centre of the mobile phone provider and confirm with "OK".
On some phones that support dual SIMS, the message centre of one card may overwrite the settings of the other SIM. In this case, check that these do not collide and are correct.
Special case #2: Number portability
When a number is transferred, it can happen that the old settings are still present in the mobile phone, even if the new SIM card of the new provider has already been inserted. Some mobile phones do not seem to update the settings correctly because the phone number has not changed, even if the SIM card number is different. Here, too, it is necessary to check the settings of the messaging centre.
Disruption to your mobile phone provider
There may be current faults with your mobile phone provider. Please check the website of your provider or portals such as https://xn--allestrungen-9ib.de for corresponding messages.
In public places or in places where there are a lot of people at the same time, the mobile phone network may be overloaded, which can lead to delayed SMS reception. Solution: Please try again at a later time in another location.
Check ensure that :
Have sufficient available memory on your mobile phone.
The SMS messages are stored locally on your device, as SIM cards only have space for very few messages.
Spam filter/SMS blocker
Spam filters can delete SMS unnoticed or move them to the spam folder. Privacy/Security Apps can block SMS from identityTM or Namirial if the senders are wrongly classified as potentially suspicious. Please check your settings.
Virtual mobile phone numbers (eSIM) that only work via app (e.g. fraenk, second phone, Numero, etc) can also be a problem when sending SMS.
WLAN telephony can also be a problem. Although providers promise that SMS will work perfectly here, this is not necessarily the case. (For example, o2 did not activate the feature for SMS reception via WLAN telephony until the beginning of 2022. Practical experience is only now being gathered).
SIM card defective
Check whether you can make phone calls. If not, and there are no current faults with your mobile phone provider, the SIM card may be defective. Try it out in a second device to check the functionality.
Defective mobile phone
The other way round, there may also be a problem with your mobile phone. To rule this out, insert another SIM card - if available - into your device as a test and restart it.