Russian wiktionary, викисловарь, displays it as [tʲɪlʲɪˈfonːɨɪ̯]
телефóнный - first two e's are reduced to "ɪ" like usual, stressed o, geminated (held for two beats) н (doesn't really sound any different in real speech honestly), and unstressed, good old adjective endings

I have heard the Russians often adopt foreign words with E sounds, and spell it with an "е", regardless of palatalization. They have a strange aversion to э, which is *supposed* to be the hard indicating version.

' for stressed vowel:
компьютер - pronounced компью'тэр

интернет - pronounced интэрнэ'т

Both телефон and телефонный however are pronounced as spelled, in terms of hard-soft consonants. This is probably because they come from very very very old Greek "tele"+"phonos" (inaccurate spelling and forms, apologies, but the idea stands)