It was a setting in my GSM provider network
windows mobile 6 pro with l2tp/ipsec --> over GPRS --> Cisco ASA
The tunnel is set up and the windows mobile pro device can be pinged by devices behind the ASA. The moment I try a tcp connection from the mobile device to a machine behind the pix the connection breaks.
So I measured the mtu to be 1270. Setting tcpmss to 1200 and the interface mtu to 1270 still doesn't help. I am thinking to make this a TAC call. I am too close to give up.
Your help is appreciated!.