Most phone companies keep landline phones somewhat active even when they have been shut off. This is especially true in appartment complexes. This allows them to do much less work the next time a phone is activated in that appartment - they only have to activate the switch port and associate it with the correct number. You may find that you get a dialtone if you plug in a phone. You won't be able to make or receive any calls though (although they often still allow 911 calls to go through or even calls to the operator to request a service activation).
Some phone companies also are happy to use any "spare" pair of wires when the normal ones are in use or fail to pass checks. This is especially common in apartment complexes that allow phone service through multiple providers (e.g. the "phone company", alternate phone providers - and I'm not talking about the VOIP folks like Vonage here, cable TV companies, etc.) Alternate phone providers are more common in appartments than in single family homes since they can split the cost of getting their service over many customers.
Even if the line isn't at all active, it's still next to lots of others that are active and may have quite a bit of noise flowing on them, especially on unterminated wires like you're looking to use as an antenna. Some of these services may use various RF frequencies to provide the various types of service provided by the phone company, or used by them to reduce the number of physical pairs needed to provide these services.
Just be aware that while most of the telephone service use rather low voltages, some can pack quite a punch. Talk power is generally provided around 48 VDC, although phones may work well with much less to allow for the voltage drop over the rather long distances using thin wires. Ring voltage, however, is around 90 volts at 20 Hz, which can give you quite a jolt if you get across it by mistake. There may also be other power on some lines since the phone company may have installed some "wall wart" power transformers for various reasons. While no longer common, it's good to be aware of this and check prior to using any pair that may normally be "spare".
