E1756 only outgoing voice works

[jabz10]

I have a Huawei E1756 enabled for voice with Mobile partner 16.001.06.01.500
I can make outgoing calls fine and can talk.
But I cannot receive incoming calls. The calling party hears a Busy tone.
The software doesn't alert of incoming call.

I am using this on a 3G only network that supports UMTS850.
There is no GSM on this network, Telecom NZ. However outgoing calls work fine.

Do you know what the issue is?

I have queried the modem via Terminal
AT
OK
AT+GMM

E1756

ATI6
OK
Manufacturer: huawei
Model: E1756
Revision 11.126.07.10.100
IMEI: 355849033935595
+GCAP: +CGSM, +DS, +ES