How to convert .h264 files into mp4 container

Hello,

due to the lag of the video playback within the voicemail function, I would like to convert the messagexxxx.h264 files together with the .gsm files into mp4 containers to be playable with e.g. vlc player.

When I try to use ffmpeg to put the raw h264 into an mp4 container (as a start), I get only errors and not streams are found:

ffmpeg -i msg0017.h264 -vcodec h264 -acodec aac output.mp4

h264 @ 0x56032133f180] Format h264 detected only with low score of 1, misdetection possible!
[h264 @ 0x560321340d00] missing picture in access unit with size 7805550
[extract_extradata @ 0x560321341380] No start code is found.
msg0017.h264: could not find codec parameters
Input #0, h264, from ‘msg0017.h264’:
Duration: N/A, bitrate: N/A
Stream #0:0: Video: h264, none, 1200k tbn
Output #0, mp4, to ‘output.mp4’:
Output file #0 does not contain any stream

What did I do wrong please?