No spam calls as soon as I transferred my own number to them. And they have mostly fixed this now anyways by assigning a random number to their SIMs upon activation rather than having a fixed number range.
North America got carried away with the banning of Chinese companies which started with the whole Huawei thing. I'm not personally concerned about dealing with these companies but everyone has their own opinions and approaches to safety. I appreciate the name China Mobile will put a lot of people off.
Just had a quick look over on TrustPilot. It appears the transferred message is when they tried to call their own number during an issue with their number being ported. If their phone number is in a black hole during a split port fault then it could be getting routed literally anywhere but it will only be routing within the UK. While you're connected within the UK a CMLink SIM is processed domestically using EE's own systems, servers and APN. Only fairly basic details are handled over in Hong Kong for the purpose of customer service as far as I am aware.
As for what pushed me towards them - CMLink's customer service seems to be more responsive and proactive than 1P Mobile. The 24/7 support with Asian values is something I appreciate. Similarly the online account website feels a lot more like it belongs in 2024 unlike 1P's who's is painfully ugly and dated. And at the time, the packages that CMLink had available were better value than 1P for what me and others in my circle were looking for.
If you're happy with 1P Mobile and have concerns over the security of China Mobile stick with what you know. But if you fancy a change then I don't really have anything bad to say about them.