While it's true that the very nature and definition of Bluetooth is a pairing of two devices, you can pair any single device to multiple devices and as long as you're not trying to do the same thing on two devices simultaneously you should be fine.
For example, streaming stereo audio utilizes the A2DP Bluetooth Profile while talking on a phone uses the HSP Bluetooth Profile. So you can be streaming your music to your Jabra BT530 from one device and when you get a call, it should automatically switch over to the phone device (i.e. you can't stream stereo audio while you're on a call - not only would it be difficult to hear the caller, but it defies the definition of Bluetooth).
You shouldn't have any problems listening to music through the Bluetooth FM transmitter and taking calls from your cell phone when both are paired with the Jabra BT530. I would suggest pairing the iPhone first, then the Bluetooth FM transmitter. The iPhone would try to pair using both profiles but the A2DP Profile would then be "taken" by the transmitter.