Use how to use a telephone when you're describing or asking for guidance on operating a phone. Grandma asked me to show her how to use a telephone. The phrase fits the moment someone is learning the device, not an active call.
Choosing your wording by situation
Use how to use a telephone when someone is learning to operate the phone, and you're asking for or describing that instruction. Could you show my father how to use a telephone with a touchscreen? This tells the help desk you want teaching, not a call.
If you're naming a class or session you're preparing, the phrase works as a topic label. The community center offers a short session on how to use a telephone. The wording stays neutral because you're naming the subject being taught.
If you're writing a note arranging help, the phrase fits as the thing to be taught. Please give Grandpa fifteen minutes on how to use a telephone while I'm at work. The note assigns instruction time, so the phrase reads naturally.
Mid-call, drop the phrase and name the contact goal instead, since the person you're talking to is already using a phone with you. Let me tell you how to reach me sounds right; how to use a telephone would puzzle a listener already talking to you on one.
When the phrase sounds off
Ask one question before choosing the phrase: is the phone the thing being learned, or just the channel you're already using? If someone's learning the device itself, how to use a telephone fits: The volunteer showed the new residents how to use a telephone.
The wording flips when your goal is reaching someone rather than operating the gear. Mid-call, the phone stops being the subject, so the phrase drops away. Say Let me tell you how to reach me or I'll show you how to call the office. The moment you're giving contact or dialing guidance inside a conversation, you're past the phrase.
If you're unsure which situation you're in, ask whether a beginner holding the phone could follow your sentence. If yes, keep how to use a telephone. If your listener is already on the line with you, replace it with a calling phrase instead.
One boundary to keep in mind: the phrase describes teaching someone to operate the phone, so it belongs in sentences about instruction time. It fits a note like Please give Grandpa fifteen minutes on how to use a telephone while I'm at work, where the minutes are spent showing him the device. It does not work as a way to talk about the content of a phone call itself; if you're already speaking on the line, you're past the phrase.