At my local office there are 2 lines. 1 for those with SS card issues (75%) and a 2nd for everybody else. The ones in line are all for line 1.
I've had major--major screw up from those at the 800 number. When you walk in you can have them date stamp so you have a record, thus my preference for walking in. The local office has screwed up as well, but with names and records, issues can be corrected.