The i355 is actually a cell phone, and while it does have two way radio capability it may not necessarily function solely as a two way radio.
According to the manual, it uses MOTOTalk, which is Motorola’s proprietary system of ten pre-programmed channels and 15 privacy codes. The manual does not specify what these channels are, but based on the general description and the notices such as “MOTOtalk may not be offered by your service provider” and “MOTOtalk is not compatible with older Family Radio Services products”, I surmise that the i355 uses a type of FHSS (Frequency Hopping Spread Spectrum) technology on the 900 MHz band, which is what the Motorola DTR radios use.
The difference is that the i355 is a cell phone with some two way radio capability, a small, consumer grade antenna and consumer grade battery pack. The DTR radios are business class and dedicated digital two way radios, with business grade antennas and longer lasting batteries. While both may use the same spectrum, the DTR radios are more powerful, hence their range will likely be greater than the i355.
In addition, according to the manual, the i355 requires a SIM card and cell service to activate, and since MOTOTalk is part of that service, it is likely that unless you activate the service, those phones may not work as two way radios alone at all.