Why is a phone call making a DNS query?

Is this related to Softphone / VOIP in any way?

The article covers it, but to reiterate. Yes, The idea is to have a mechanism to map phone numbers to hosts.

The normal path for voip phones given a phone number is to end up at a sip trunk provider go over the traditional phone network (which at this point probably routes over the internet anyway) hit another sip trunk and end up at the receiving sip phone.

This provides a method to bypass the traditional phone network and go directly over the internet. The sip phone looks up the host responsible for that phone number and directly connects. The sip providers would be responsible for maintaining this number to host mapping in dns.

When you think about it DNS is really just a big distributed phone book, a key value store to look up numbers based on names. The reverse records are a method to look up names based on numbers using that same distributed architecture.

There is also an interesting legacy architecture interaction here, traditional phones can only enter numbers. Cell phones could use dns names directly(but don't) or we could use ip addresses as a sort of modern phone number(but don't), The whole world was connected via phone numbers and that is now how we expect phones to operate.

If the phone is behind a NAT firewall / router it may need external help to receive calls: https://support.aa.net.uk/VoIP_NAT