After you log in. Go into your account settings and change the number. If this doesn't work, call the Trulia number at 888-466-3501 Monday - Friday 7am to 4pm PS. and ask them to change it or walk you through it. I hope this helps.
Claire is correct. The phone number displayed on your listings is pulled from your Trulia profile, so you'll want to edit your profile to change it. To do that, access the link below to navigate to your main profile page. Then, select 'Edit Profile' on the left. Under the 'Personal Information' section, you should find the option to update your phone number.
Let us know if you need any further help. You can always feel free to contact our Customer Service team either by phone at 888-466-3501 Monday - Friday 7am to 4pm PST, or by email via http://www.trulia.com/help/ask/.