telefonieren
to phone
To have a conversation with someone over the phone.
Ich kann gerade nicht, ich telefoniere.
I can't right now, I'm on the phone.

to phone
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to phone
To have a conversation with someone over the phone.
Ich kann gerade nicht, ich telefoniere.
I can't right now, I'm on the phone.

The verb 'telefonieren' (to phone) is derived from the noun 'das Telefon' (the telephone). The ending '-ieren' is a typical way to form verbs from nouns or foreign words.
Ich muss nachher noch kurz mit meiner Mutter telefonieren.
I still have to phone my mother briefly later.
If you want to say whom you are speaking with, always use the preposition 'mit' followed by the dative. A direct connection with an accusative object is incorrect.
Gestern habe ich über eine Stunde mit meiner besten Freundin telefoniert.
Yesterday I was on the phone with my best friend for over an hour.
'Telefonieren' (to phone) is often used to signal that you are busy. The answer 'Ich telefoniere gerade' (I'm on the phone right now) is a polite way to say that you can't speak at the moment.
Kannst du später wiederkommen? Ich telefoniere gerade geschäftlich.
Can you come back later? I'm on a business call right now.
to speak
'sprechen' and 'telefonieren' can both mean that two people are having a conversation. The difference is in how the conversation takes place. 'sprechen' is more general: you can speak with someone directly, in a room, at school, or at an appointment. 'telefonieren' is used only when the conversation takes place over the phone. Learners may confuse the words because when you are on the phone, you also speak. If the phone is important, 'telefonieren' is more precise; if it is simply about a conversation, 'sprechen' fits.

Kannst du später mit dem Gast telefonieren?
Can you phone the guest later?
In der Pause spricht Frau Fischer mit den Eltern über die Reise.
During the break, Mrs. Fischer speaks with the parents about the trip.