If it's for the smartphone brand for the big blob, from my experience, most will still go over the general promos available to current customers, even after declining to provide a phone number.
I remember one shop though, where the associate insisted on a phone number. I gave him an old number linked to a corporate account with the carrier. He said I would have to ask the admin of the plan. I told him that I'm switching jobs and considering switching over to a personal plan, and asked him if he could tell me the promos for both current and new customers. After, he became more open to providing it w/o a phone number.