On daf 21a Levi says that although the benei Bavel are eating (or about to eat) on Yom Kippur because they didn't know the correct Rosh Chodesh, he could not testify to the correct day since he had not personally heard Beis Din's announcement of Mekudash.
On 22b, Ulah states that in Eretz Yisrael they had been mekadesh the Levana, and the Gemara broadens this to rule that anyone, not only Ulah, could make that statement because it's a milsa da'avida leg'luyeh.
Do Levi and Ula disagree or is there another factor here that Levi could not, and Ulah could, make an authoritative statement on Kiddush Hachodesh.
Gershon Dubin
1) The Rambam Hilchot Kidush Hachodesh 3:14 hints strongly at the answer to this question when he writes:
"Even a peddler from the rest of the people who comes and says ' I have heard from Beit Din that they sanctified the moon, on such and such a date' he is believed'".
This suggests that Ula did not merely say that they were mekadesh the moon in the West, but he actually said that he personally heard the Beit Din say "Mekudash". In contrast, Levi said specifically that he did not hear this.
2)(a)However the Ritva (end 22b) writes that when the Gemara there cites a Beraita which states "even if somebody came from the end of the world and said 'the Beit Din were mekadesh the moon' he is believed'", the intention of this Beraita is to inform us that even "Eid MiPee Eid" is sufficient for this testimony. Even if the witness who said that Beit Din sanctified the moon did not claim to have heard it himself from the Beit Din, but rather he heard from someone else who in turn heard the Beit Din say Mekudash, this is sufficient.
(b) According to this, we will have to say that not only did Levi not hear directly the Beit Din saying Mekudash, but he also did not hear from anyone else who had directly heard this from the Beit Din.
(c) However there is still a problem here. The source of the Din of Eid MiPee Eid is in Yevamos 122a, that even if the first witness heard from a second witness that the husband had died, this is sufficient to allow the wife to remarry. The Be'er Heitev on Shulchan Aruch Even HaEzer 17:13 writes that even if there is a chain of 100 witnesses, this is also sufficient. If we say that this din of 100 witnesses also applies for Kidush Hachodesh, it is somewhat diifcult to understand why Levi's testimony was not sufficient. Levi knew that Beit Din had mekadesh the month. He presumably must have known because he heard people saying this and they in turn must ultimately have heard this from soembody who heard it from the Beit Din itself.
(d) I do not have a clear answer to this problem but there are a few possibilities (1) the chain of 100 is not sufficient for Kidush Hachodesh (2) possibly Levi only heard a rumour about Kidush Hachodesh, but did not hear from anyone specific, and this is weaker than Eid MiPee Eid.
3) To summarize, according to the Rambam we have a clear distinction between Levi and Ulla and according to the Ritva also it may be that Eid MiPee Eid did not apply in the case of Levi.
KOL TUV
Dovid Bloom