Differnces between 2106B and 2106+

We have an embedded platform using the Wavecom 2106B GPRS modem on the Optus network in Australia. When we swap the 2106B out and replace it with a supposedly backwards-compatible Wavecom 2106+ (the successor to the 2106B) using exactly the same parameters and SIM, we get a PDP authentication failure.

Does anyone have any ideas please ?