What frequency band are you targeting with the jammer? I am not going to claim to be an expert but here are my thoughts.
The 555 timer may not be able switch at a high enough frequency for the fundamental switching frequency to get into the GSM or CDMA bands to jam a call. The harmonics may be high enough to be in those bands but the phone spread spectrum broadcasting would probably allow it to still work.
Second the bandwidth of the output stage may not be high enough (~2GHz) at least to achieve the amount of needed power to jam a signal (80mW).
If you choose an amplifier that has the right amount of power and bandwidth you can over came this second issue.
I like minicircuits.com for amplifiers.
Also I came across a jammer design on the web here:
http://electroschematics.com/1003/mobile-cell-phone-jammer/I uploaded the picture as well.