Is this message considered proof of a minor irrevocable divorce (baynunah sughra), or is it merely evidence of an unwillingness to reconcile, and does the husband's right to take back his wife (raj'ah) thus fall away?

The return (to the marital union) does not require the wife's consent or knowledge, by scholarly consensus. Nor does it require witnesses according to most scholars. Thus, a return via phone without witnesses is valid according to most scholars, and the wife must return to her husband. The return does not require a guardian or a dowry, because it is a retention of the marital state and a continuation of the marriage. It is, however, recommended to inform the wife of the return for fear that she might marry someone else, mistakenly believing her 'iddah has ended. Witnessing the return is not obligatory according to the Malikis, Hanafis, and the new position of the Shafi'is, in contrast to those who deem it obligatory, citing the verse: "And take as witness two just persons from among you." They consider the command for witnessing to be for recommendation.