It's not surprising that this doesn't work, as phonebook entries don't have punctuation.
One thing to look at is whether IP calling is enabled:
Settings->Call Features->Disable Direct IP Call
Though that is no doubt set right since you seem to be able to dial manually.
Another possibility is to enable "quick IP mode", which lets you dial just the last part of the IP address, which wouldn't require punctuation. Enable this (it's disabled by default):
Settings->Call Features->Use Quick IP Call Mode
and see if, for the IP address 192.168.1.57, dialing 57 or 057 works, then try putting those digits in the phonebook. It might work. I'm too lazy to try it myself, but we all would appreciate hearing how you make out...