Trying to fix it so I can use my gift cards

I like this app but I tried to get a hold of customer service and I can't remember how I got a hold of them email and they haven't responded back My number I only have one phone number and I think someone used my phone number and it's giving me an error saying my phone number is already in use and I only have one phone number how could I get this fixed could someone let me know I appreciate it thank you