I suspect I might know what this is, I am not 100% but it sounds like it is a good probability.
You might want to check what the phones send time is. That is the number of seconds before the phone has decided you have finished typing the number and its now ready to send. If this is set too low you need to raise it so you don't have to be as quick typing the number in.
Unless both the phone and the PBX support the 484 error message (Address incomplete) then what usually happens when you dial a number is that the phone gives you a predetermined amount of time to dial each key in. If you exceed that time the phone decides you have finished typing the number in and its time to send it off to the PBX for connection.
If your phone supports immediate send then you can actually switch this on and each time you enter a digit the phone sends it to the PBX straight away to see if an extension or number exists. This actually removes the need for the delay as the PBX will simply say "Not yet, keep trying" (Or 484 address incomplete) until the complete number has been entered and will immediately connect you.
The downside is Axon does not support this....yet. So for now the only option is to use the delay. Increasing it though should solve your problem.