If somehow you find yourself in an unfortunate situation where a conversation just putters down to an awkward near silence, and seemingly stuck, simply take a deep breath and relax... avoid vocalizing with 'uhhh' and 'ahhh's just to keep some noises going, silence is sometimes okay.

Think back whether you've given enough time for your partner to speak, and ask them what they thought of the conversations you've just had, or their day, or the weather.

Or you can simply look down and tinker with the n900 that you've been holding with your two hands on your lap and ignore the other party.