I use the Jawbone. With my phone, it is not instantaneous. If I am on a call and I want to switch to headset so that I can type while talking, it will take 2 - 4 seconds to activate the headset and another 4 - 6 for my phone to recognize it and for me to "accept" the recognition.

Usually, I will put the earphone on, activate it and watch the handset. When I am asked to accept, I in turn ask the person I am speaking to hold just a sec and indicate acceptance of the head-set/hand set relationship. After that it is a just second or so.

In short, your phone will have something to do with how seamless it is to activate the headset.

I do love that Jawbone.