The key to understanding Num 11:29 is actually V27 and the way the verb נָבָא (naba = to prophesy) is used.
- Num 11:25-27 - people prophesied when the Holy Spirit came upon them
- 1 Sam 10:5, 6, 10, 11, 13 - people prophesied when the Holy Spirit came upon them
- 1 Sam 18:10, 19:20, 21, 23, 25 - Saul prophesied (against his will) to stop him killing David when the Holy Spirit came upon him
- 1 Kings 18:29, 22:8, 10, 12 - false prophets (of Baal) prophesied in an ecstatic state
- Joel 2:28 - a prophecy that most people will be able to prophesy
In all these cases, there is an associated comment that when a person prophesies under the complete control of the Holy Spirit, they become a prophet. This verb is always used of a verbal communication.
In the NT, the idea is subtlety different - prophets are those that still deliver verbal messages such as those from Agabus and the four daughters of Philip; but there is an added dimension - prophets are also those who wrote written messages, including that which ended up in the Bible. This is understandable as in the early days of the Bible, the concept of the Bible was nebulous and scarcely formed.
However, there is a definite continuity: Joel 2:28-32 is quoted by Peter in Acts 2:17-21 with the same meaning. There is no reason to suggest that this meaning changed between Acts 2 and 1 Cor 14:5.
In 1 Cor 14 - Paul hopes (vainly) that all people would prophesy but (presumably) realizes that is not his decision but that of the Holy Spirit. However, that does not prevent Joel 2 being fulfilled at some future time.
There is some debate about what the NT meaning of "prophesy" meant - one or more of three possibilities exist:
- "prophesy" was a loose synonym for "preaching" (there is little evidence for this)
- "prophecy" is a purely verbal message as per Agabus, Silas and the four daugters of Philip
- "prophesy" in the sense of contributing to the canon of scripture as did Paul, John, Jude, James, etc.
Further, there is nothing to prevent more than one of these being true simultaneously (the most likely). In any case, most of the "prophesying" appears to have been verbal and did not contribute to the canon of Scripture in the NT, much as was the case in the OT.
However, as 2 Peter 1:19-21 makes clear, some of the prophets' written messages were canonized in Holy Scripture for all time. This means that (probably most) prophesying was verbal and never recorded and thus had a purely local application. This did not make it any less inspired, just inappropriate for inclusion in the Bible.