[Serusers] When is a DNS lookup performed?

When does SER perform DNS lookups? Is it a matter of how the entry is made in the ser.cfg? I ask because one of our customers changed their DNS servers and the SER proxy was not updated. In the ser.cfg all routing is done by IP (or I had thought it was). However, the site started experiencing some very odd call drops and after updating the resolv.conf on the proxy server with the proper DNS servers (one of them had changed) everything was fine.

Should IP address entries be quoted or non-quoted? Does the presence of the quotes cause a DNS lookup even though the item in them is purely numeric (i.e. "162.128.33.54"). Is there a way to avoid the DNS lookups?