Did not have that mmsprovis.conf (due to automatic setting).
Set it to manual and file appeared.

After reading my settings and comparing to your settings above the only ppssible misconfiguration I see might be the port.
you wrote in your settings
mmsc-port:
proxy-port: 8799

but provider tells only about
mms-port:8799

so maybe set
mmsc-port:8799
proxy-port:

But I am not convinced!

Other thing might be the size of MMS.
To what is it set?
Set it to <= 300k
Maybe provider is blocking bigger messages (should not, only charge higher fee).

Else no idea, sorry.


--
just read you already tried that port thingy
Is your network set to 3G or dual?

--
what is the error output?
try dmesg
or
tail /var/log/syslog