The locate handset button will make the missing handset ring for 60 seconds. This will usually give you time to find it (assuming the handset battery is good, the handset is not out of range, and/or the handset ringer isn't set to OFF). Once you have the handset, press the Page/Handset Locator button or place the handset on the base.
You can press the locate handset button again if you haven't found the handset in one minute. The manual for the Vtech 2451 is available from Retrevo: http://www.retrevo.com/support/Vtech-t2451-Telephones-manual/id/470ag516/t/2/
. You will need to enter a code to download the pdf.
I hope this helps.
(If you have a large area or several floors to check, those 1 minute intervals aren't enough time to search for one person. Enlist assistance if you suspect the missing phone isn't just under a throw or pillow near the couch (for example).)