It's smooth to connect to Telstra IoT network using Arduino MKR NB 1500 with fantastic Tesltra support.
We have observed a few problems.
(1) After MKR NB 1500 runs for a couple of hours, the modem may fail to work and it cannot connect to Internet any more, unless a cold restart (power off/on) imposed;
(2) At deep sleep mode, MKR NB 1500 still consumes around 1.5mA, which drains the battery heavily and is not good for long running sensors;
(3) The IoT network coverage is indeed bigger than mobile phone network coverage. MKR NB 1500 can still connect to IoT network at the place where my telstra phone has no reception, however the signal is marginal (RSSI around -100dB). The connection and data can get lost sometimes.
I'm so glad to hear that you have been enjoying the IoT network with Arduino's dev kit. Thanks for raising those issues.
For the battery inefficiency, other devs have found a way around it by upgrading the UBLOX modem on the device to the latest firmware, see this dev with a similar problem: https://dev.telstra.com/content/faq-arduino-mkr-nb-1500
Please check the IoT coverage map and if you think the device is in a clear place with no obstructions (i.e not underground or inside) and in a designated coverage area and you still have issues with the signal strength, please send us an email with the company details you used to purchase this device on TelstraDev, including the device IMSI and SIM ICCID: firstname.lastname@example.org