This is the 6180th most frequent Russian word.
телефонный
"Телефонный" means "telephone" or "phone-related."
In this sentence, 'телефонный' is used as an adjective to describe the type of the 'звонок' (call), indicating that it is made through a telephone.
Телефонный звонок разбудил меня рано утром.
A telephone call woke me up early in the morning.
Here, 'телефонная' describes the 'сеть' (network), emphasizing that the network is related to telephony.
Я нашёл телефонную сеть с лучшим покрытием для путешествий.
I found a telephone network with the best coverage for travel.
'Телефонной' qualifies 'линии' (line), specifically referring to it being related to telephone communication.