We have had a number of customers report that they are unable to connect to MaxoTel over the Optus Wireless 4G network.
This is an issue with the Optus network interfering with SIP traffic, which affects both MaxoTel and other providers.
Unfortunately there is no known workaround for this issue at this time.
The recommended solution is to switch your 4G connection to another provider, such as Telstra, which has been shown to work.