Wait, are you saying when a call comes IN to the system from a number that matches the System Speed Dial directory, that the name isn't substituting properly in the display of the phones?
Verify the number is the exact same... if your carrier is sending +12345678910 and your entry is 2345678910, it doesn't match.
Also, I think we had a thread about this a few weeks ago, seems to me it was decided this is just a SIP Trunking issue but I can't find the post.