Client/Server ppp over GSM


I have a problem with the ppp server. When I receive a data call login it´s ok and the connection is established.

After that, if I make ping from the PC to modem,it responded, but one or two responses properly, stops.
In case of opening a client socket, after the first data, the ppp connection looks good, but no data flow throurgh the socket.

Someone with a client or server on GSM working properly, can give me some light?
The same code works perfect if instead of starting a GSM client/server, stars a GPRS conection or UART ppp server.