"From what I read, serial communication to a modem is possible without CTS and RTS, but not recommended since it is less reliable.  I was going to try it myself, but I wasn't willing to even try if it's known from the beginning reliability may suffer."

"I don't know one way or the other.  The mini is sold with a 4-wire cable to connect the 4 pins of the minit to the six wires of a legacy telemetry module.  A module that does have CTS and RTS pins while the mini does not.  So I assume RTS and CTS…"

"I second this.  From what I gather, the mini lacks RTS ands CTS pins."

"I second the above, schematics or more information is needed to hook a RDF900+ up.  Customer support only has the manual which has no technical information.  They even went so fr as to say: "...All you need to know about the Pixhawk mini is on the…"