Iphone 7 to Galaxy S7 calling Issues

My brother has an Iphone 7 on T-Mobile, and every time he tries to call me it says call failed. We've always been able to text each other without issue, and I (who owns the galaxy s7) can call him without issue. Its only when he tries to call me with the IPhone 7 that it says call failed. This issue is only with my phone number, and I don't know of any other person who has issues calling my number. Anybody run into a similar issue?