There's a cost/benefit analysis to be done. People do make mistakes, and checking their submission against common patterns will save you lost sales. As suggest

There's a cost/benefit analysis to be done. People do make mistakes, and checking their submission against common patterns will save you lost sales. As suggested in a comment on a rant I wrote a few years ago, one strategy is to 'validate' against something reasonable, but instead of rejecting anything that fails, ask the user to confirm that they really do want something unexpected. I'm not sure that 'reasonable' would be for an international phone number, but whatever you pick based on the country they choose, let them override your validation.