Hi a was looking about a problem (https://supportforums.cisco.com/thread/2058182) and I found a strange bug with this modem and this version of IOS:
857 Cisco IOS Software, C850 Software (C850-ADVSECURITYK9-M), Version 12.4(15)T14, RELEASE SOFTWARE (fc2)
I always use the "ping -c1 -M do -s 1472 xxx.xxx.xxx.xxx" from a linux box connected to the cisco adsl 857, so with this bug and the use of -c1 option, you get always bad result.
Cisco just solve this issue in the:
857 Cisco IOS Software, C850 Software (C850-ADVSECURITYK9-M), Version 12.4(15)T15, RELEASE SOFTWARE (fc3)
If you have the same problem, maybe this could help you...
Best Regards