I've just read through the WADA rules, and am justifying it by giving you all a brief summary.
The key documents are as follows:
ISTI:
https://wada-main-prod.s3.amazonaws.com/resources/files/WADA-2015-ISTI-Final-EN.pdfTesting guidlines:
https://wada-main-prod.s3.amazonaws.com/resources/files/wada_guidelines_effective_testing_2014_v1.0_en.pdfResults guidelines:
https://wada-main-prod.s3.amazonaws.com/wada_guidelines_results_management_hearings_decisions_2014_v1.0_en.pdf
ISTI is the International Standard for Testing and Investigations and has the actual rules, then the the Testing guidelines and Results guidelines give details on how the process should be run.
For ISTI Annex I is the relevant portion, starting on page 84.
An athlete gets 2 chances to respond before the missed test becomes official, however I.5.5 makes it clear that when an athlete hits three missed tests they can challenge any of the previous tests, even if they were not challenged at the time. Further, should Armistead have a fourth missed test, she can challenge test 2 or 3, even though she didn't challenge them previously.
Moving on to challenging a missed test, what's striking to me is how much flexibility the DCO (Doping Control Officer) has in making their own judgement on what to do, with "the only universal guideline being common sense" (9.2.1 of the Testing guidelines). The key thing they have to prove, is that they made a "reasonable attempt" to locate the athlete. The DCO is not meant to inform anyone of who they are, until they see the athlete however, and I'm quoting the below because I suspect it's what Armistead relied on:
"In certain circumstances, a degree of advance notice may simply be unavoidable. For example, an Athlete may live or train at a location where access is controlled by security personnel who will not permit access to anyone without first speaking to the Athlete or (for example) a team official. This in itself is neither improper nor suspicious, but the DCO should be especially vigilant in such cases of any other circumstances which may be suspicious (such as a long delay between the security guard contacting the Athlete or team official and the DCO being given access to the Athlete). In this case, the DCO should provide a full report of such suspicious circumstances and should consider requiring the Athlete to give a second Sample."
The other point to note, on the phone call is the following:
"If the Athlete does not answer, the DCO may telephone the Athlete to advise him/her of the attempt in the closing five minutes of the 60-minute period. Such a call is not mandatory however, nor should it be used to invite the Athlete for Testing, but rather to potentially further validate that the Athlete is not present."
Annex K (page 100) of the Results guideline document then provides more detail on what Armistead had to prove during the appeal, which is probably the following:
"The DCO did what was reasonable in the circumstances, given the nature of the specified location, to try to locate the Athlete during the 60-minute time slot, short of giving the Athlete advance notice of the test. That the Athlete’s failure to be available for Testing at the specified location during the specified 60-minute time slot was at least negligent. In accordance with ISTI Article I.4.3, negligence is presumed on showing that the Athlete was not where he/she said they would be; and to rebut the presumption of negligence, the Athlete has to show that no negligent behavior on his/her part caused or contributed to him/her (i) being unavailable for Testing at such location during such time slot;"
The key point here is probably whether what the DCO did was reasonable, and it's impossible to know without more detail, however given the first quote above, I suspect that the defence that they didn't identify themselves to the hotel staff should be enough.
The guidance does give a few pointers which might point against Armistead (from 2.4.3 of the the results guidelines):
1) It isn't an acceptable excuse to not hear a doorbell ring. By the same virtue, I would assume that not hearing your phone ring isn't an acceptable excuse.
2) The guidelines state that if the location is a sports-complex, then the athlete should specify where in the complex they can be found. By the same virtue, should an athlete have to confirm which room they are in, if they are at a hotel?
Finally, there is no guidance on what a DCO should do if an athlete is staying at a hotel. That strikes me as a massive oversight, which needs to be corrected, given the inevitable challenges that exist in trying to reach an athlete without giving them advance notification. (Err, mandate they give a room number in ADAMS?)