That is to do with the Cisco 7970 there, not the PBX. You'll need to log on to it to check what the dialplan is configured as. Sometimes they'll come with a default that cuts off digits and such.
Ok
It seems strange to me that it only happens with the extensions that start with 511 if from that same terminal I mark 5100 it is processed without problems
I'm still going to check the template of the phone
Without knowing what the dial plan in the phone is, it is mostly a guess but there is probably some form of "X11" in it to capture the NANP special services short codes. 511 is typically for highway information.