I've used a similar (900 MHz ISM band) walkie-talkie system from Motorola.
Not the Motorola DTR radios, but the "DirectTalk" off-network walkie-talkie feature which is included as a freebie on a lot of recent (2006 or 2007 and later) Motorola iDEN handsets as used by Nextel, SouthernLINC and a few local cellular carriers.
DirectTalk uses the same architecture as Motorola's DTR walkie-talkies, but I don't know if they are compatible, or intentionally blocked by software from interoperability. Basic version: 902-928 MHz ISM band, frequency-hopping spread spectrum, digital audio, 10 "channels" and a "private call" function that allows one to make a call to the telephone number programmed in the receiving handset.
DirectTalk is not to be confused with the DirectConnect on-network walkie-talkie service offered by iDEN callular providers, which uses the same frequencies and towers as cellular phone service.
DT will work even on inactive iDEN handsets, the sort you can find on Ebay or Craigslist for under $10, so long as it's a DT capable handset: Motorola i275, i670, i776, i335, i355, i560, i570, i576, i365, i580, i880, possibly a few others; also on the Motorola "PowerSource" CDMA-iDEN handsets they build for Nextel: ic402, ic502, ic602 and ic902. Some Boost handsets are supposedly capable of using DT also, with a minor software tweak.
The handsets require a SIM card be installed in order to boot-up, but it doesn't need to be an active SIM card, or even an iDEN SIM card. And they allow the "phone number" assigned to the handset––-that DT uses for "private call"––-to be set by the user.
I've used a pair of i355s, fitted with the fixed antennas Motorola made for the i325 handsets, at ranges of over 6 miles. The 900 MHz ISM band is very much line-of-sight, so terrain and vegetation will have a major effect on range. It may be possible to configure a couple of handsets to function as a simple low-cost duplex repeater: set one to the input channel and route its output to the second handset, using a basic repeater controller module, on the output channel, add a car battery or other power supply, and site them on a hilltop or tall structure to give maximum coverage. "Private calls" sent to the input handset's programmed number would then be retransmitted to other users in the area on one of the ten virtual channels.