I'm having the same problem. My settings match my provider. Sent a report to Samsung and all I'm getting back is "check if you have blocked numbers.." and such.. why on earth would I be texting a blocked number 🤔 stupid.. recently upgraded as I like android..but Samsung are just garbage. My settings are correct My call centre number is correct, as its the same sim from the last phone. Some texts to people will go through, but can then just decide to not send them for no reason...