All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
We have already clarified that the bank is one institution with all its different sections and that it is not permissible to work in any section of a bank that deals with Riba; rather this ruling even includes the security guard and so on.
The fact that you do not conduct Riba (interest and/or usury) transactions does not exempt you from responsibility. For instance, you are the one who is responsible for the team who handles the phones which are used to fulfill the Riba-based transactions, so there is no way to escape responsibility.
If the situation is as you mentioned, that you asked whom you trust in regard to their religion and they issued a Fatwa to you that this job was permissible, then you are not sinful for acting according to that Fatwa. However, since it has become clear to you that working in that bank is forbidden, then you are sinful to work afterwards and the money that you get in return for that work becomes forbidden (ill-gotten money) for you.
Nonetheless, we can not say that in this case your sin is greater than the sin of the one who abandons the prayer because abandoning the prayer is an act of disbelief that may take a person out of the fold of Islam, nor can we say that its sin is more serious than committing Zina (fornication or adultery). But its sin could be greater than the sin of the one who shaves his beard as no scholar has said that it is permissible to cooperate in sin.
In any case, it is not important to know whether this sin is greater or minor than other sins, what is important is to know that it is a grave major sin and that it is an obligation upon you to repent to Allaah and immediately give up working in this bank that deals with Riba unless you are in a dire necessity to an extent that you do not have any earnings and that you fear that you and those on whom you are obliged to spend will starve or be harmed and that you do not find another lawful job to earn your living.
For more benefit on the prohibition of working in a Riba-based bank because it is cooperating in sin and transgression, please refer to Fatwa 141008; and to Fatwa 85917 about the ruling on the money obtained from a Riba-based bank; and to Fatwa 142583 on the ruling on working in a Riba-based bank in case of necessity.
Allaah Knows best.