I'm looking at SAR values for phones and find that it's often very difficult to find published values or even the FCC ID of the various devices. That said, what I have found published for the REVVL is encouraging to someone (like me) who is looking for a device with minimal SAR emissions, especially at the head (i.e., when used as a phone). With the caveat that FCC documents simply provide the manufacturer's own test reports, I approached the issue with the hope that the tests were conducted properly and the results were provided in good faith.
As someone who is considering switching to T-Mobile, the new REVVL looks interesting (although the lack of Gorilla Glass and water-resistance, and the non-removable battery are drawbacks). I found that the "head" SAR rating for the REVVL published on PhoneArena https://www.phonearena.com/phones/T-Mobile-REVVL_id10627 is an astonishingly low "0.20 W/kg". But heading over to a different site http://www.devicespecifications.com/en/model-sar/5da544e7 yields a reported head SAR of "1.18 W/kg", more in line with a lot of other GSM phones. The discrepancy bothered me and I tried to understand it.
In the case of the REVVL, the FCC ID is available (2ACCJB089) and I think that the FCC document "I17Z60508-SEM01_SAR_Rev0_part1" found at
https://apps.fcc.gov/oetcf/eas/reports/ViewExhibitReport.cfm?mode=Exhibits&RequestTimeout=500&calledFromFrame=N&application_id=U6VqFyNBWu3kNBpoCMYg8w%3D%3D&fcc_id=2ACCJB089 explains the discrepancy: the head SAR tops off at 0.20 for the GSM emissions but is 1.18 for WLAN (2.4GHz) emissions - something that I guess can be avoided by simply leaving wifi disabled when the device is being used as a phone.
For comparison (at a similar price point) The Motorola Moto E4 (model# XT1762) can be found at https://apps.fcc.gov/oetcf/eas/reports/ViewExhibitReport.cfm?mode=Exhibits&RequestTimeout=500&calledFromFrame=N&application_id=inEQv1njG2UDs6CBzGuHqQ%3D%3D&fcc_id=IHDT56WC6, and the document "Test Report RFExp Part 1" reports SAR values as high as 0.52 W/kg for GSM transmission with WLAN at 1.10 W/kg.
While I prefer a non carrier-branded phone like the E4, with the expectation of timely software updates and the absence of "bloatware", I think I'm going to give the REVVL a try.