Range and performance in Direct Talk (DT) mode on the R765 should be identical to the DTRs. The DT feature is the same FHSS digital platform that the DTRs were derived from. With DT mode in the R765 or any other iDEN phone with the feature, you take a step backward from the DTRs in terms of features. They are based on the MOTOtalk platform and the DT feature appears to be an early implementation of MOTOtalk. The modern implementation of MOTOtalk is today’s DTRs and the DLRs. The DTRs (and DLRs) use the exact same FHSS system (MOTOtalk) but were coded differently from the DT feature in NEXTEL phones. The DT feature had to be kept simple for phone users. Its closest relative is the DTR410 in terms of features. I suspect the DTRs became incompatible with DT mode when public and private group features and text messages were added to the DTRs. The MOTOtalk name is referenced in the certification filings with the FCC for the DTR410 (FCC ID: AZ489FT5842) and the DTR550/650 (FCC ID: AZ489FT5852). I also recall seeing correspondence with the FCC documenting the name change from MOTOtalk to DTR.
MOTOtalk article on Wikipedia:
https://en.wikipedia.org/wiki/MOTO_Talk
One gotcha to be aware of is in order to use the DT feature, an iDEN phone such as the R765 or an i355 needs to have a SIM card installed that was previously activated on an iDEN network (NEXTEL, Southern Linc Wireless, etc.). This is because the phone number that was put into the SIM card when the phone was activated is used to form the 11 digit Private ID for the DT feature, consisting of 1 plus the area code and phone number. The DTRs (and DLRs) have their 11 digit Private ID assigned to them at the factory and follow a 1 plus an area code (717) and 7 digit phone number format. The MOTOtalk platform is an ID based system and requires each device to have its own 11 digit Private ID in order to work.