There are a number of reasons why you would want to trace an IP to a phone number. Although the process of tracing an IP is arguably an easy one, tracing it to a phone number requires more effort. The IP owners subscribe to a confidentiality and privacy code that prohibits them from giving out their user's contact details.
Investigate the header of the email whose IP you want to trace. Log in to your e-mail account using your username and password, and then open the specific e-mail to start the tracing. Display the email headers by using the properties or details functions of the email program. Look for the "received from" properties, which will indicate the IP address between square brackets [ ].
Use sites dedicated to tracking IP addresses to find further information about the owner who has been assigned this particular IP address. There are free websites that serve as apt resources to get more details on an IP address. You can use American Registry for Internet Numbers, dnsstuff.com, tracemyp.com, urgentclick.com, among others, to find contact information on the owner of the IP address. (See Resources.)
Type in the IP number into the search box of any of the above-mentioned sites and click "Submit." Note that it is not possible to track an IP to the phone number of the person who has been assigned the IP address or the person who has sent you the email. The user's ISP is the only one who can tell you a user's phone number and other contact details.
Contact the user's ISP in the event that you want to report malicious e-mails, unsolicited mail or spam. Consider that to trace an IP to a phone number will require the concerted effort of the ISP provider, a court order and authorisation from the police.