I have seen references to this issue in a closed post of 2017 but no resolution. Not sure if it is a UK Phone Co-op network issue (as suggested in the 2017 post) but sporadically (including right now) text messages to a few contacts fail to send; I get the failure message ‘number incorrectly formatted.’ I have checked and the numbers are fine - +44, no spaces or errors. I regularly send to these contacts and in fact sent messages to them successfully just a few hours earlier today. I have changed nothing since.
Right now messages to other contacts send perfectly which suggests to me it isn’t a network issue. It happens every few months (and it is not always the same contacts affected) and does not last days but is still a nuisance when you need to send a message to someone and can’t. Any ideas of fixes very welcome!