On occasion, 3CX servers may automatically blacklist the MaxoTel SIP server. This has been observed as an issue arising in recent version of 3CX.
The solution is delete the blacklist entry and add a whitelist entry for the MaxoTel SIP server address.
This may be related to DID's being routed to your 3CX PBX from the MaxoTel Portal, which aren't configured in 3CX.
3CX identifies these as unauthorized calls and as result, blocks the SIP server after a number of attempts.
If this is the case, we recommend ensuring all your DID's are properly configured in 3CX, as well as following the steps below.