Post 70 911 may have existed in Dallas but not in the sense we know it today. A three digit number was certainly possible because 411 was Information. Dallas could have had 911 as a short cut to the police to save time going through a TELCO operator. TELCO’s back then also had customized two digit prefix numbers to call into another nearby phone exchange without it being a toll call.
http://inventors.about.com/library/inventors/bl911.htm Feb 1968 was the first 911 system and it would have been simply an answer center. E-911 came later.