I think that if a conversation by phone reaches the stage where neither of you have anything left to say, it might be better to just say thanks and end the conversation, especially if she's the one calling and it's costing her. With face to face conversations, there's no need for this. It's not costing her anything to leave a silence hanging, and if you two are really close it shouldn't matter if you don't always feel like talking.
I wouldn't describe you as boring. Cautious, perhaps, and tend to be conservative in how you express yourself. I wonder whether you hold back what you're really thinking sometimes. Whether someone is "boring" is highly subjective. If she didn't find you boring to begin with, it's unlikely that will change unless you put no effort into the relationship at all. Not being extroverted and gregarious is not the same thing as being dull.
Just my two cents. Honestly, it's not something you need to worry about.
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Let's find that exit they call paradise...