Hello Chief Inspector Cain here. The investigators should get the names of those being ripped off, match these names to the teller(s) used, they might find a pattern. Also, check those who are involved with customers making large withdrawals such as Managers who would usually have to sign off on such withdrawals.
No personal phones to be used during work, that is what breaks are for. Next step, get permission to bug the phone lines.
Hard to implement, not practical and even illegal. Bugging phones will, and should, not be allowed.
Use the internal surveillance cameras to see which employees the customer interacted with and track those. Do a set up using police. When you nab the criminals check their phone records as to who the communicated with. Then you can work back to the employees’ phones.