I think in a previous thread (and judging by your username) you said your daughter was going to Corpus. One piece of advice I can give is that the phone signal in the main accommodation block for first years was notoriously terrible (when I lived there in 2020/21), I barely knew anyone who could get service in there! Luckily the Wi-fi in there is fairly reliable so there are many ways to keep in touch over the internet.
I use giffgaff which worked great in the rest of the city. Its pretty cheap, sim only, and the monthly rolling version allows you to switch plans if you need more/less data and pause the monthly plans altogether (which may be useful if she doesn't need the UK sim if she goes home to the USA in the vacations?). All the plans come with unlimited UK texts and calls too. I'm not sure how good they are for international calls/texts but to be honest you'd be better off keeping in touch over the internet (e.g. Whatsapp calls and texts) rather than using mobile service calls and texts as that can get pretty expensive.