Наш is used with masculine nouns, наше is used with neuter nouns.
There’s also наша, used with feminine nouns, and наши, used with plural nouns regardless of their gender.
The gender of the noun is not always immediately obvious and need to be learnt for each noun separately. But once you learn enough Russian, you’ll be able to make educated guesses about the noun’s gender (e.g. most nouns in -а and -я are feminine, most nouns in hard consonants are masculine, most nouns in -о, -е are neuter).
«Телефо́н» can refer to all types of phones plus to a phone numbers.
If you need to disambiguate them, you could add an adjective:
- «моби́льный телефо́н» (mobile phone) or «со́товый телефо́н» (cell phone; the latter is becoming less popular),
- «стациона́рный телефо́н» (landline phone; literally 'stationary phone'),
- «но́мер телефо́на» (phone number; literally 'number of phone') or «телефо́нный но́мер» (literally 'phone number', less common).
For mobile phones, there're a colloquial word «моби́льник»; alternatively, you can just drop 'телефо́н' and say «моби́льный» ('mobile'), this will also be understood to mean 'mobile phone'. (You can't drop «телефо́н» in «стациона́рный телефон», though!)
Not sure which other types of phone exist? 'Smartphone' is called by a loanword «смартфо́н», for 'feature phone' we don't really have a conventional name, I usually just say «обы́чный телефо́н» (ordinary phone) or «не-смартфо́н» (non-smartphone).