Testosterone isn't really all that an effect birth control agent, which is why it was rejected by the World Health Organizaton as an option. On the other hand, progestins like Deca are much more effective in lowering sperm count.
This is why the new formulations of male birth control pills include both a progestin and a form of testosterone replacement. The progestin is for fertility reduction and the testosterone is for maintaining sexual function and mood.
The effect is only temporary. When you go off it, your fertility should go back to normal. If it's just testosterone I wouldn't worry too much.