Someone else , more familiar with your particular gateway, might want to chime in here...
The dialtone you were getting, after you "sent" was probably related to the Stage Method, the gateway was using. You want One Stage, as the gateway will seize DT then pass on the digits from 3CX. Wait for DT is not always necessary with modern digital switches. Other than a dead line, it's assumed that dialtone will be there by the time the handset reaches your ear on almost all PSTN switches. Delay before dialling should also not be crucial, but, there might be issues if it is set too short.
Check the 3CX log to see if the call is being passed to the gateway, if 3CX is having a problem doing that. If the gateway is not registered, and 3CX cannot pass a call to it, that would be the first thing to try to correct. To check out registration problems, power down the gateway, power up, then immediately check the 3CX log for errors, that might point to an IP or password datafill problem.