This should be an issue by default, in my opinion. That said it's not why I distrust Huawei (not that I trust Google, Cisco, Qualcomm all that much more - but I give them more benefit of the doubt). I don't even care about ATT because if you trust your carrier you're already toast, and I do not know any carrier that I respect as a company (despite them having great employees).
Here's why I distrust Huawei and similar companies:
From my 20y incident response experience in the field at very large companies, we've never caught the NSA or similar US agencies. Either because they're too good, we're unlucky, they're too bad at it, or because they don't actually need to, or do not backdoor. Who knows.
What does bother me is that we caught other countries several times, sometimes using terribly bad tactics (where the defense was bad for various reasons that are usually also bad) and sometimes using relatively novel stuff that we'd never see before.
Thankfully you do not have to trust me, ask any security engineer at any big company (amazon, fb, uber, google, lockheed, etc.). They'll either say they're not allowed to speak about it or give you an answer that's similar to the above (or break their NDAs, alternatively, I guess?!)