Twitter can handle north of 4k tweets per second on virtually entirely open source software and these morons can't record phone calls. If this is not a glaring example of government mismanagement and waste I don't know what is. Where are the criminal indictments?
911 call centers have to interface with many different computer systems (for example the National Criminal Information center, department of motor vehicles, FBI CJIS, in house databases, court records systems, mapping, computer aided dispatch, weather information, etc.
Many of these systems (the NCIC, and CJIS have very strict requirements on what can and can not connect to their systems, and on top of that who can use a terminal that connects to their systems).
Its not an easy task. However comparing it to twitter is just ... wow