sending from mobile device message gets hung in gwia

I have a strange problem in that sometimes my outgoing messages get sent ok from a mobile device other times it just seems to hang in gwia and on the mobile device. (i.e. the connection seems to hang).

IF I turn on airplane mode on the phone then turn the airplane mode off, the message gets sent ok. (When I do this my mobile device gets a new ip address and the message gets sent ok.)

I turned off /RBL checking as I noticed my cellular carrier has some blacklisted ip addresses, which I ocassionallly get.

Any idea's on what I can do? I also checked the blocked.txt file and there is no ip addresses in there.

Thanks