It wasn't that uncommon for residential burglar alarms to use polarity reversal on dry leased lines, but it was quite expensive. In a series circuit like this,

It wasn't that uncommon for residential burglar alarms to use polarity reversal on dry leased lines, but it was quite expensive. In a series circuit like this, I'd more likely guess that it was dedicated wiring owned by the alarm company. This was fairly common practice in some markets and one of the reasons that the burglar alarm companies were so often also in the telegraph business (e.g. ADT). Of course AT&T could have done the same thing, and indeed they tried, but their burglar alarm ventures were never that successful. Except when they owned ADT for a while, but the fact that they bought ADT and continued to pursue their own alarm division underscores that they had a hard time with the alarm market.