Most common problem why the sms fails to be sent is the wrong "SMS Service Center" number. This number is stored on your sim card and can be changed from your phone, the setting is usually found under settings for sms/text messages.
The number actually comes loaded when you get your new sim card, but if it gets accedentally edited or deleted, sending SMS fails because of the wrong SMS service center number. The number may have been changed accidentally on your phone. Some apps also give a quick access to this and you might have changed it unknowingly. If you use the sim card with usb or pcmcia modem/data card, the software you use on your pc may change the service center number based on what operator/country you chose at the time of installation/use. Correcting the number with the right one should solve the issue.
SMS service center number is different for different operators. So find out the sms service center number for your operator from their website or customer care and make sure that the number in your settings is correct.
Some phones support multiple profiles for SMS sending, in such case, the SMS service center number of the profile in use need to be correct.
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