[Freeswitch-users] No ringback for calls from cell phones

Hi, I am using FreesWITCH with Sangoma 102 and E1 connection. When I make a call from a fixed number to one of the internal extensions, ringback can be heard on the fixed number. However, when I make a call from a cell phone, although the internal extension rings, I cannot hear any ringing on the cell phone. I have tried two different mobile phone operators, but the result is the same. I have tried specifying the channel variables such as ringback and transfer_ringback int he dialplan, but doesn't help. Any pointers on how to debug this? Could it be the telco, in between the cell phone operators and FS, causing this? Thanks!