It may not be your favourite idea but after having tried multiple times it may be wise to go into the store in person. An explanation to someone in management, or preferably the individual that interviewed you, is potentially your best option. Do you have the name/personal details/department details of the interviewer? If so, mentioning them or attempting direct contact could help. The only downside to this could be the person you want to speak to not being in store. However, the person you do speak to may take note of your situation and pass it on. Certainly don't give up with calling though, there may simply be a temporary issue with the store's phone line. I hope this helps!