Because the response will generally be faster.
If you call 911 from a campus landline phone, or 972-860-4290 from a cellphone, it will ring directly to DCCCD police dispatch. If, however, you call 911 from a cellphone, it will connect you to the local municipal police department in your area. City officers are not as familiar with college buildings as campus officers. If, for example, you call your campus police and say you need help in room L035, campus officers know exactly where you are. So, they generally are able to respond more quickly.
Be prepared for emergencies: program the campus police dispatch phone number,
972-860-4290, into your cellphone.