Unfortunately, it seems that Todd is correct.

This isn't a function of the phone, but of T-Mobile. When my carrier was
Sprint, and I wanted to send a fax directly from my laptop and was away from
a phone line, I could hook up the Sprint phone to my laptop, and use it as a
modem to send faxes from my laptop.

My phone with T-Mobile (Motorola V60) also can be hooked up as a fax modem.
Windows recognizes it as a fax modem when I connect it to my laptop, and I
can connect to the Internet that way. (As CSD, calling my regular ISP,
counting as minutes from TM, not as MB via GPRS.) However, when I tried to
send a fax through that modem there was always an error message. I called TM
tech support, and the rep told me that to be able to send faxes through the
TM phone I would have to pay $10 per month extra and get an extra phone line
just for faxes. I found this incredulous, and thought he might not have
known what he was talking about, as is often the case with hired
phone-answerers. So I called again, and spoke to someone else. Each time
(sometimes after they left me a long while to research it), I got the same
result.

As with Todd, that was something I would only do occasionally, so it
certainly isn't worth my adding $10 a month to my TM bill for. (And you are
still billed for the minutes thus used, besides that additional $10
monthly.)

I really wonder whether anyone uses that feature enough to make it worth
their while to spend $10 a month on. I doubt many people sign up for that
service. I would think that TM would do better not to have that extra
monthly charge for that capability. Then more people would do it, use up
more minutes, and perhaps have higher bills due to the additional minutes.
With that extra charge, I doubt they have many people on that plan.