Is there a word or phrase to describe the action of replying to communication via another medium? For example, if someone sends me a message containing a question via Facebook and I reply to it via a SMS text message. In this case the person asking the question, gets an answer, but may be confused by it because of the medium for the reply, or because the chosen medium may lack context for the response.
Messages not on the original or main channel are typically labelled out-of-band.
For example, from Wiktionary:
relating to communication on a different channel, or by a different method, from that of the primary communication channel
and again from The Slang Dictionary:
in communication, using a medium other than the current one.
For example: discussing some finer point via email, rather than clogging a discussion forum.
But note that this phrase alone does not describe replies, only messages sent outside a particular topical medium, so get very specific you'd have to say your SMS message to your FB friend was an out-of-band response.
Note the "responding via email, rather than clogging up the discussion forum" example is also sometimes described as taking it off-line.
This latter phrase is also generic, and can, for example, be used of a conference call or a large meeting, if a couple of participants find a topic which needs to be discussed but diverges from the main topic of the meeting, where they can meet independently, outside of the meeting.
Backchannel in this context is a response through another medium, and is usually described as having one of three purposes. Wikipedia describes the substantive purpose as
Substantive backchannels consist of more substantial turn-taking by the listener and usually manifest as asking for clarification or repetitions.
This may or may not be exactly what you are referring to, but I would interpret using a backchannel to mean that the user of the backchannel felt that the primary medium is not appropriate for the type of response being provided. (Traditionally, it might be disruptive if made on the primary medium, but in this case there may be other reasons. For example, it might involve personal or tangentially distracting information that isn't appropriate for the primary channel.)
Another definition supporting your question is
backchannel - a secret, unofficial, or informal channel of communication as used in politics or diplomacy (Dictionary.com)
You often hear that diplomacy is carried out through a backchannel, but a backchannel is not limited to secret communications. I think this definition supports the notion that a backchannel would be used when the primary channel or medium is considered inappropriate for some reason (see examples above), and an alternative medium is available.
Regarding the confusion that you mention, Wikipedia says this:
Confusion or distraction can occur during an intercultural encounter if participants from both parties are not accustomed to the same backchannel norms.
This is not in popular usage, but what about "multi-midial disconnect" or "multimedial disconnection." It implies the different forms of communication along with a potential misunderstanding which might stem from it.
If you're not concerned specifically with the "misunderstanding" part of your query, you could also use "multimedial communication" or "multimedial exchange/chain."
You can call it mixing it up or changing platforms/formats/modes/lanes.
"Today is so boring, I am going to mix it up and do something different."
To best effect at work, I shift platforms when the first format gets no answer. When a coworker was frustrated that no teammates got back to his formal email for one-on-one meetings, he asked about resending the email. Instead, we mixed it up and just walked over to see people in person without more emails that didn't get through. No one said no in person. Similarly, if a phone call falls flat, send an email or a handwritten note.