Ask HN: How do phone spammers know my location?

For the past month my cell phone has been receiving the same robo call from various phone numbers. This is normal. What's new is that the calling number changes based on my physical location. I took a trip to Massachusetts and the spam calls were Massachusetts numbers. I spent a night in Washington DC and the calls changed to Washington DC area codes. How does the spammer know which phone number to "call" from to match my location? Is this likely a malicious app, are websites tracking my IP address, or is there some other path for spammers to behave this way? How could I even find out how this information is leaked?