On a whim, I bought a Monoprice bluetooth headset* since it was only $17 and I was ordering some other stuff anyway. I've never used one of these things before. I paired it with my iPhone and leave it in the car. I find that it doesn't always seem to be 'connected' when I get in the car. Is it normal for the two to take a couple of minutes to find each other?
Also, holding the button is supposed to initiate the voice command mode on the iPhone. This doesn't seem to work consistently for me either although maybe it is related to the problem above.
Depends on the device; my Mazda3 with built-in BT takes at least 10-20 seconds to pair my iPhone with the system after the ignition is turned over. Other devices may take longer, and it likely depends on the product quality as well (and if it possess proper support for the iPhone). I don't have a definite answer for you.
__________________ ACMT MacBook Pro (15-inch, Mid 2012) 2.3 GHz Core i7, 16GB, 256GB M4 SSD iPhone 8 - 64GB S/G • Sound System Audio Engine A2 • Display UltraSharp U2412M 24" Custom Built Gaming PC Kaby Lake Edition | 16GB DDR4 | GTX 1070 8G
Have you thought to contact the customer service or support forums for the makers of your ear piece?
It most likely has to do with the power management your headset is doing. Many headsets are passive and tend to put themselves to sleep until the master needs to communicate. There could be a problem with how your device talks to your iPhone or master device, it could be interference, or it could be your iPhone.
I don't know if there is an "official" bluetooth compatibility list, but this appears to be a good start: