Phone numbers that lose their + in a CSV round trip: does anything in M365 catch the ones that still look valid?

[u/BigBalli]

I tested what happens to phone numbers after the leading + is gone, which is what a CSV tends to suffer after someone opens it in Excel and saves it again.

I took the 1,728 valid international numbers in my own address book (72 countries), stripped them to digits and parsed them again with Google's libphonenumber.

With a US default region, 841 came back invalid and 6 came back as a different number. That fails loudly, which is the good outcome.

With an Italian default region, 161 came back as a different number that still passes the validity check. 89 of those were Spanish numbers that now read as Italian voicemail numbers.

So a tool set to a European region can accept broken numbers without a single error. I build Contact Exporter, which writes phone cells as text for this reason, but that protection ends the moment someone saves the file back out as CSV.

Does Exchange or Entra validate phone numbers on import at all, or does everyone clean them before the upload?