Sorry, I have no info about this.. there is no “phone specific” parameters, so either the E/// phones are faulty (surprising !) or all phones do actually have this problem in your network… then it might be because parameters of your nw are misconfigured.
Sam what is your network equipment are you using? Ericsson handsets are not having trouble in other people networks. Have you looked at the SIMs you are using (if any) with E/// handsets?