I remember talks here on HN about Chinese made IOT devices phoning home to servers in China. There have also been discussion on HN about Huawei's telecom equipment being backdoored or simply rigged to siphon information back to china.

Lots of people talking about Huawei backdoors, none providing credible evidence of them actually existing. They have their share of security vulnerabilities but no worse than competitors such as Cisco[1].

Random Chinese made IOT devices phoning home to servers in China is definitely a thing, but has nothing to do with Huawei.

[1] https://www.techrepublic.com/article/evidence-of-backdoors-i...

If this were the case, why do the vast majority of mobile phone networks around the world continue to use and even buy more from Huawei?

Naiveté and a bit of corruption and/or blackmail. More people need to watch this recent interview with a former CCP insider on the Chinese threat: https://www.youtube.com/watch?v=4cwXifDaCjE&t=3s.

What kind of data is being sent?

Devices that need to legitimately phone home for something tend to do so by requesting servers within or close to the country they were developed in.

Imagine people getting riled up at the thought of Apple phoning home to a US-based NTP server..