Simple Mobile uses the T-Mobile network.
It doesn't necessarily (or even likely) follow that Simple Mobile calls would count as T-Mobile-to-T-Mobile calls. The T-Mobile terms of service
say: "T-Mobile-to-T-Mobile calls are those made between T-Mobile customers using their Devices while on the T-Mobile network." The section saying T-Mobile-to-T-Mobile calls are "those made between T-Mobile customers" would be unnecessary if being on the T-Mobile network were enough. I tried to find some evidence T-Mobile to Simple Mobile calls were billed as T-Mobile-to-T-Mobile, and didn't find any.
He may not have known the difference between Simple Mobile and T-Mobile.
I think BDLR probably can tell "Simple" from "T." I think someone who's not tech savvy would be less likely to confuse the two, because they wouldn't know that Simple Mobile is a T-Mobile reseller.
All and all, I think T-Mobile to Simple Mobile calls did
get billed as T-Mobile-to-T-Mobile, only because that seems the least unlikely of unlikely alternatives.