You can't call ahead, and you can't just walk in. Prepare for a long line especially if it's cold. Once you get to the front of the line, you can either get tickets for the next available tour (they sell out fast) or purchase tickets for a future tour. Watch out, since the beer hall is one-in / one-out, and the tour groups all take up spots in line for the beer hall. I suggest going as soon as they open and get in line to get a tour ticket for your time of choice. Then when you return at your tour time, you can jump the line! The line is the worst part about this place.
You have to walk in - but the line to get in can be long so try to arrive early or dress warm to wait outside! You buy tickets for the tours once inside and I had no trouble getting on the next tour of the day (every 20 min)
Just wanted to add- Prepare for a long line *typically only* thurs-sat, and in the evening. If you go earlier in the day it's not so bad. Does get slammed when Blue Hills has a popular show, however. Source: I work at Harpoon and there's an observable pattern!
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