What is a dry pair ?
Generally, a dry pair is a pair of telephone wires that does not have a dialtone or battery on it.
Dry pairs are often used as dedicated lines for data applications and internet communication such as SDSL, ISDN, or a T1 service.
In some rare cases, dry pairs/loops may have a dialtone (Bell Canada) used to call the telco that identifies the number you're calling from.
Note: When one requests a dry pair for data applications, the line should also be "unloaded" by the telco. What that means is, they should remove any load coils from the existing cable (used to balance the freq response for voice frequencies). Those load coils also block higher frequencies used by T1 and DSL lines.