You'd be the expert on this, but I'm skeptical of your claim that a single transmitter could be identified because that would mean the communication code is leaving behind extra bandwidth.

But voice calls aren't secure because the metadata can't be obscured and the content probably wouldn't be encrypted. So, yes, a pay phone or disposable prepaid mobile would provide more anonymity for that, but the reason we'd need a smart phone like the Neo900 is if a Wifi connection isn't accessible and we'd like to transmit non-voice data whose metacontent can be obscured with something like the TOR network. So there'd be no:
Call to 001555990000077
Call to 001555123456789
Call to 001555221111111
...just a PXS8 modem with a given IMEI connecting to TOR from a given location. Could be another Neo900 or even another type of phone or maybe even 10 different ones leaving a busy train station and maybe 10 more on the return trip.

There's a difference between the 47 bits of the IMEI and the 1 bit represented by the question "PXS8 or not PXS8?" and I already have my carrier and my government telling me how many bits of my privacy I do or don't need to protect.

Without the Snowden revelation I doubt the Neo900 would be as popular as it is. People want to reign in control over their own lives and data and that happens 1 bit at a time. 1 bit is supposed to be all it takes to keep someone out of jail, but since people are getting sent without trial to Gitmo nowadays we need a few more.

IMO anonymity is relative: more vs less, not yes vs no.

Although I wouldn't confuse the amount of conditional entropy of the IMEI with its absolute entropy, it suffices to say that every bit I manage to successfully withhold is an extra bit the NSA needs to get via compromising TOR (easier) or by building a quantum computer (harder).