das Telefonat
phone call
A conversation conducted over the telephone.
Ich muss noch ein wichtiges Telefonat mit einem Kunden führen.
I still need to make an important phone call with a customer.

phone call
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phone call
A conversation conducted over the telephone.
Ich muss noch ein wichtiges Telefonat mit einem Kunden führen.
I still need to make an important phone call with a customer.

The ending '-at' turns the word stem 'Telefon' (telephone) into a noun that describes a single event or process. So it refers to a single telephone conversation.
Das Referat des Studenten war sehr informativ.
The student's presentation was very informative.
Especially in formal or business contexts, 'Telefonat' (phone call) is often combined with the verb 'führen' (to conduct): 'ein Telefonat führen' (to make/conduct a phone call).
Ich muss später noch ein langes Telefonat mit der Versicherung führen.
I still have to make a long phone call with the insurance company later.
To name the person you are speaking with, use 'mit' + dative. To name the reason, use 'wegen' + genitive.
Ich hatte gestern ein kurzes Telefonat mit meiner Chefin.
I had a short phone call with my boss yesterday.
conversation
'das Gespräch' and 'das Telefonat' both have to do with exchange between people. 'das Gespräch' is more general: it can be in person, professional, casual, or formal. 'das Telefonat' is more specific and always means a conversation on the phone. So you can say: a phone call is a special kind of 'Gespräch'. The words can be confused if you are only thinking about the content. If the medium is important, meaning that it was by phone, 'Telefonat' fits better. If the type of exchange is the main focus, 'Gespräch' is more natural.

Der Mitarbeiter führt gerade ein wichtiges Telefonat mit einem Kunden.
The employee is currently having an important phone call with a customer.
Nach dem Gespräch mit der Chefin fühlte sich Anna viel sicherer.
After the meeting with her boss, Anna felt much more confident.