Is it a “telepone”? No, it’s a telephone.
So it’s not “dip-theria,” and it’s not “dip-thong” either. Diphtheria. Diphthong. Look at it.
The same goes for ophthalmology and ophthalmologist.? Oph-thal-mol-o-gy.? Oph-thal-mol-o-gist.? There is no such thing as an “optomologist.”