MP3 is a poor engineering choice, at least with default parameters, as it is over-engineered fro the actual quality of phone audio.
.wav with a gsm payload (= wav49 or WAV in Asterisk), will produce the smallest files for a universally supported format. Note that the .wav49 extension will not be recognized on windows, but it will be recognized with .WAV (or .wav).
Asterisk .wav will have the most universal support.
(It is possible to encode MP3 at lower bit rates than GSM, but I don’t know how many devices can handle such low rates, or how the quality compares. I would expect GSM to be better for speech than very low bit rate MP3. The default bit rate should produce file sizes similar to Asterisk .wav, with no quality improvement, and a lot of CPU processing.)