This is usually associated to the DTMF tones not being understood correctly by the system. This, of course, assumes that the input at the phone does indeed correspond to a valid name. You did not indicate if the issue is observed using a particular type of phone so -
Use a landline analog phone and test, this should be the baseline and all should work well
Use a cell/mobile and test. All should work well, but if not then see if the settings of the cell allow for a timing setting of the DTMF tones (some do and perhaps set too short)
Use a IP-Phone and test, if one works they should all work provided that they are all set to use the same style of DTMF generation and duration (look in the phone using its web interface and you will see settings)