Is there a way to retrieve the inbound phone number? I assume the "Dynamic Variables" is used for outbound calls?

You can retrieve the inbound phone number from the customer.number field in the assistant-request payload and use it in the variableValues to personalize responses in Vapi

can this be used in a workflow?

I failed to do this in workflow. call id 04b73577-5c22-4d64-a083-f2fe1295b0ff . the retrieved inbound number is empty when using {{customer.number}}