Do you know an alternate feature code for account code ?
You can change it yourself to what ever you want in the Feature Access Codes. Just choose a number that is not going to conflict with things like your extensions, system speedcalls, external calls and reception or any other feature access code.
so for example if you use 0 to dial external, 9 for reception and you have extensions 1000 to 3999 and system speeddials 4000 to 4999 you could try picking something like 88.
at first just work on testing, and then if the number is not suitable you can change it later but atleast you have an idea what to do.
Other thing I have found on occasion is that some sip stuff have digit map in the settings, I know on my polycomm units before I removed the digit map if i hit the call button before dialling it worked, but if i dialled then hit the call button the dial string would determine it as invalid.