When you are responding to customer email that is the result of a series of messages sent back and forth, you may want to occasionally refresh your contact information by including it in your most recent response.
This will help your customer find your phone number or other contact information more easily, should they wish to use it. Otherwise they may have to scroll all the way down to the beginning of the email stream to find your contact info.
This may be even more important for Blackberry (and similar device) users. In many cases, Blackberry users cannot scroll to the beginning of an email stream since the Blackberry is limited on the length of the message it supports – much of the original traffic may have been truncated.