According to the article, by default Signal will relay all calls from people who aren't contacts. This means if a non-contact calls you or vice versa, they can't see your IP address and you can't see theirs, even if the call is accepted and you're both talking. They also provide an option to enable relaying calls to/from contacts, so that contacts won't see your IP address, either.
Here, regardless of if you have that setting enabled or not, and regardless of if you accept the call, contacts and non-contacts can cause your device to make a DNS request, which will leak your DNS server. And if using a DNS server with EDNS Client Subnets, the first 3 octets of your IP address will also be leaked.
I think there's another issue like what you're describing which can kind of obviate this, though: the vast majority of Signal users probably use Signal on their regular mobile phone and its number, not a burner phone/SIM/number. (Few users probably even own a burner phone/SIM/number or understand what that is or why they might want one or how they'd obtain one.) So... everyone can just see your phone number, which probably has an area code corresponding to your city or close to it, and the other digits can possibly pinpoint it even more precisely than that.
Anyone who isn't tunneling all of their DNS traffic with a VPN or otherwise probably also isn't anonymizing their phone number and just has the app installed on their personal, standard cell phone.
If they aren't traveling and haven't moved recently, you can probably see what city they're in just from that. (This exposure does allow coarse location detection even when someone's traveling, though it's a lot more coarse than the area code, unless the Client Subnet value is being sent.)
