Phone transfer during call to external #

I didn’t see a section for phone so posting here. 

We have a need to transfer a call to another phone number for another business unit. 

Use case: 

1. call comes in
2. call answered
3. realize it is for other business unit who do not use Intercom
4. need to transfer to that phone number

We don’t see a way to do this.
I can transfer a call in a workflow, before it gets to the agent.
Anyone solved this?
Zendesk supports this.