Interface telephone handset
This may work okay, depending on the impedance of the mic (which unless you have the capsule datasheet there is no way of knowing)
Make sure you are using the microphone rather than the earpiece though. The earpiece  impedance will be different and will not be as sensitive.   

In any case, let's make sure you have the connections correct. 
Connect pin 2 ("hot") XLR input to one pin of the mic, and pin 3 ("cold") to the other. Connect pin 1 (cable shield) to the capsule case. Use the mic input, not the line input.
Let us know how it goes, if it's still bad you will probably need some passive/active impedance matching, which is easy enough.  



The above is assuming it is a dynamic capsule. If it is electret (or carbon as Jippie mentions, although I doubt this as it looks quite a modern handset) then it needs a bias voltage to work properly.
It should be possible to tell which it is by examining the capsule and surrounding circuitry (if any) If you post a couple of close up photos (front/back/surrounding circuit) we can hopefully let you know.