I have seen this before and I would think it is a dialplan issue. I am pretty sure there is a way to make it universal, so that you can dial as many digits as you need. When you pause for a few seconds (time out), the phone will know you are done pressing buttons and process the call.

I don't have an example of a universal dialplan that would allow any number of digits. Last time I had to fix this problem I just opied the dial string from a phone that worked.

The main disadvantage is that there woudl be a pause after you are done dialing, or you would dial a number and then press send.