Talking on a phone is a terrible medium for helping people. Say you want to give them a link to a resource, or a picture, then you have to IM or email them anyway.
Because using a phone in most offices is pretty disruptive for everyone sitting close by. You also have to consider if the other party is available or just in a meeting, bathroom etc.
IM everyone can get back to you as soon as it suits them without doing another “I’ll call you back” round trip.
I would agree a voice call is useful for a warm hand-off and sometimes easier to convey a message without excess typing. That is where Mumble (murmur server, self hosted) makes sense to me. Clean quality voice plus text chat for links and what-not. [1] There are public servers if you want to try it out first before setting up your private servers.
One of the issues with using the phone is that it's so difficult to record phonecalls and store them in a way that makes them useful for future reference.
With instant messaging, you can usually at least do simple searches for phrases you remembered from the conversation. With recording phone calls, you need to remember the approximate date that you had the chat and then skip through one or more audio files to try and find the information you're after.
It depends on the situation. The phone is useful when both parties have all required information in their heads already and don't have to do much referring to material or action in response to the conversation.