Log into My Account > My preferences. Under Block texts (toward the bottom of the page under Limits and permissions) click on the phone that has the restriction on it if you have more than one phone on your account. You should have the second-last radio button checked and a list of numbers displayed that you have blocked. Select the number you wish to unblock, click Remove number and then Save. Done.
You can also use the shortcode 9999 and text allow 9876543210 to it, 9876543210 being the number you want to unblock. This will add the phone number to the allow list and removes it from the block list. For further commands see the Support article.