The employee was probably right. I had both an iPhone 4S on Sprint from work and a personal account on T-Mobile. I was told I could use the iPhone 4S for personal use and had been using it for six months resulting in me paying a bill from T-Mobile but not turning that phone on in over three months. As I wanted to keep my personal cell phone number (had it seven years), I called Sprint customer service and they setup the port from T-Mobile. It took all of about 10 minutes but there were a few questions that really only I could answer to get it done; T-Mobile account number, T-Mobile phone number, last four digits of social security number, address, etc. All things needed to identify me as the owner of the T-Mobile account. Yes a store employee could have done it for you but basically he would be on the phone and standing in the middle between you and the CS rep. Not the most efficient way to get something done.
As for AT&T, I have not dealt with them but maybe they have the ability to do such changes, such as initiating number ports, in the store. Sprint does not.
I was all set to switch to Sprint yesterday from AT&T. I have a giftcard to the Apple store so I wanted to purchase the iPhone there. The Apple store said they can't port the number so I was going to have to start a new line then port my number over. The Sprint employee told me I would have to call customer service to port my number over. I really don't like dealing with something like this over the phone (as I already have a line for work with Sprint, I would need to rework my account to a shared plan) so I asked the employee if I could bring the phone in and have it done here. His response was: "The only difference is that we (Sprint) would have to call in to get the service all worked out, so you might as well do it yourself." That right there made up my decision not to switch. I walked out of the store and will not look back. Didn't even get the phone and the store service was very disappointing. This doesn't happen at the AT&T stores. Sprint should fix these lazy store employees so they don't lose anymore number ports.