How do I assign different phone numbers from Twilio to a single user?

How do I assign different phone numbers from Twilio to a single user?

I have clients from several different states and I always call from a local number to increase answer rate. For example: if I have a client in Illinois I will call from a phone number with area code are 773, then if I have a client in Florida I will call from a number with area code 321. 

So far I have only been able to add one phone number per user. How can I add several phone numbers to that same user? 

Thanks