iPhone 8 hearing problem

I have iPhone 8 . Last year I had a problem of hearing. I just could not hear the other person on my iPhone even if I have the hearing volume at maximum. I visited the service center last August and they did some servicing for which they charged Rs. 1500/- (Approx USD20). This was a 5-10 min job where they did something and the hearing became normal. Now within less than 1 year, the problem has occurred again. I am not able to hear unless I use a headphone. Please advise. Cannot keep on spending the amount every now and then.