Is it permissible for an engineer to work for a telecommunications company whose projects involve dealing with usurious banks, knowing that his work relates to network security and could potentially serve the religion?

It is not permissible to assist in usury in any way, such as writing, witnessing, guarding, programming, or maintenance, based on the Almighty's saying: "And do not cooperate in sin and aggression." If you can avoid that, then your work is permissible; otherwise, seek another permissible job. Be certain that sustenance comes from Allah and is attained through obedience to Him, as the Almighty said: "And whoever fears Allah - He will make for him a way out. And will provide for him from where he does not expect." So, fear Allah, beware of His punishment, and make your earnings pure; for every body nourished by unlawful means, the Fire is more deserving of it.