I've picked up an RTU5024 4GLTE Gate Opener to replace the 3G unit that no longer works. 

I've been playing around setting it up today, the majority of which has been by SMS - you SMS it a code, and it responds that it has received said code. This has been working fine - I can activate the relay via SMS, add numbers to the authorised list, allow any number to open the relay etc, and the unit sends an SMS back. 

However, I don't appear to be able to call the device and have it activate the relay. It goes straight to voicemail (with 2 degrees), and there is no response from the unit - no lights activating, no SMS to say the relay has been triggered..

The SIM in the unit has credit, is active and is on a plan. 

Thoughts or suggestions please?