One really easy reason to pick up the phone is when we have good news to deliver.
It doesn't need to be massive news -- it can be as simple as telling your client about a new feature that they were interested in, or that their social media presence is trending 30% higher this month, or even a call to let them know that their product or service will be delivered quicker than expected.
Positive news is so much easier and exciting to deliver, so I say, let's do it more... For the sake of better relationships :)
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Side note: they* say words make up 7% of our communication, and the tone of our voice, 35% -- so, even if these numbers are relatively true -- it would make sense that picking up the phone and hearing the tone of each other's voices would have a huge impact on the trust, rapport and connection we build with other people.
(*they: referring to Albert Mehrabian's famous communication model)