Your friends will have the option to transcribe your phone post and give the post a title by hitting the "Transcribe" link; however, each friend can only transcribe a post once. Other friends can continue to update the transcription until you provide your own transcription, at which point no further transcriptions can be added.
I'd like to suggest the following:
Your friends will have the option to transcribe your phone post and give the post a title by hitting the "Transcribe" link. Once someone has transcribed the entry, their transcript will appear in the phone post journal entry, with a "Transcribed" link at the bottom. Clicking this link takes you to the transcription page, where it is possible to review previous transcripts, and enter a new transcript. Only the most recent transcript will appear in the journal entry, and each friend can only transcribe a post once. Once you have provided your own transcription, your friends will no longer be able to transcribe the entry, though you can continue to update the transcription as often as you like.
Edit: I realized that this doesn't address the "can I edit a transcript I posted" question. How about inserting, "It is not possible to edit a transcript once it has been posted." in between the last and the second-to-last sentences?