@Alice996 There's nothing inherently wrong with a host or guest contacting each other by phone. That's why phone numbers are visible to hosts and guests once a reservation is confirmed.
There are some times when it is warranted. For instance, I ask my guests to phone or text me when they arrive in town, as almost all of them arrive by bus and I go to pick them up at the bus station. Sometimes guests are elderly and have trouble with modern tech and feel more comfortable talking on the phone.
Sometimes guests, especially those new to Airbnb, don't have their notifications turned on and don't respond to the host's messages. In that case, it's often necessary for the host to text or phone the guest to ask them to please look at their Airbnb Inbox and respond to the message they were sent.
It really depends on the circumstances. If a host or guest insists on phoning, texting, using Whatsapp, because they are want to say something rude or make arrangements that are against Airbnb policy, then for sure that is wrong and deserves to be reported.
Additionally, if a host says something "off" to you over the phone, it's best to follow it up with an Airbnb message- "Hi XX, I'm just following up on our phone conversation yesterday. Did I understand you correctly when you said.....". "XX- regarding your phone call this morning, I am not accustomed to being spoken to so disrespectfully. I was shocked when you told me ...."