This is pretty basic stuff as far as telecom goes. When you connect the set to the PBX, you should measure about 48V DC. This is normal and so is a bit of a drop if the loop is long. The alarm "line" you refer to should be connected onto a trunk port of the PBX (this is normal too). Alarm installations usually have special jacks for this. The alarm line shouldn't have anything to do with the station you're talking about.
When you dial that station's extension number, the PBX port should send 90V AC to the set to activate it's ringer. It's ok for it to be a bit less on a long loop too. It should be easy to measure with a meter and test. Just dial the extension number......the set should ring and you should be able to answer the call. Even is the voltage is too low to activate the ringer, you should still be able to answer the call just by picking up the handset.
If you're trying to connect to one of the alarm panel's sensors, then that's a different story! It's simple......you can't do it! In fact, you're likely to damage the alarm system by running 90V AC to it.
If you don't understand the concept of what I'm talking about, you might want to get a qualified telecom tech out to give you hand.