Was fired due to anxiety attack

While you cannot be terminated for having a medical condition which meets the criteria to be considered a disability (that is: it has a major and pervasive effect on one or more important life activities), you may be terminated for workplace behavior, even if caused by the condition or disabiity: in that case, you are being terminated for what you did, not for what you have or are. Whether it would be legally appropriate to terminate you due to an anxiety attack causing you hang up on a tech depends on the circumstances: did hanging up cause a problem for the employer (e.g. couldn't get important or necessary systems running in time) or was it an essentially harmless "hiccup"? Did you just disconnect the phone, or did you say anything inappropriate to them or act in a disturbing way prior to handing up? Did you take immediate steps to get them back on the phone and mitigate the problem, or did you delay in taking steps or reporting this to management, so the problem festered? Etc. Depending on the facts, it may have been reasaonble and legal to terminate you, if you cause enough of a problem or disruption. Or it may not have been, if what you did was not really much of a problem at all, and they essentially terminated you for having the anxiety attack.
You also write that "walked out the door"--if by that, you mean that you walked out of work during work hours when you should have, or worse, that you affirmatively said or did anything indicating you were quitting/resigning, the employer could consider that you resigned and/or fire you for absenteeism or abandoning your job: the obligation to not discriminate against those with disabilities does not extend to letting them simply miss work, or having to ignore words or behavior constituting resignation.
So based on what you write, the termination of your employement may be legally justified. But if you disagree, you don't have to simply take the employer's position at face value: you can contact the federal Equal Employment Opportunity Commission (EEOC) and discuss filing a complaint. If the EEOC thinks that you had a protected disability and that the company may have acted inappropriately (e.g. discriminated against or harassed you) in regards to it, given the facts and circumstances, they can investigate and recommend or take legal action if appropriate.