How many volts can a landline telephone handle without blowing?

Since the ringer voltage across the twisted pair is 120 VAC RMS the telephone can definitely handle that. Given the fact that the telephone is also supposed to survive induced surges from nearby lightning strikes to the line the telephone should be able to handle over 1000 V common mode impulses (but I don't know the exact limit). No telephone is expected to survive a direct lightning strike to the line (which could be millions of volts).

Note: common mode means the voltage is applied between the twisted pair lines (both at about the same voltage) and ground. If the survivable common mode voltage were to be applied across the twisted pair instead it would severely damage the telephone.