Do you know what codec you are using?
G.711 - 64 Kbps, sample-based Also known as alaw/ulaw
G.722 - 48/56/64 Kbps ADPCM 7Khz audio bandwidth
G.722.1 - 24/32 Kbps 7Khz audio bandwidth (based on Polycom's SIREN codec)
G.722.1C - 32 Kbps, a Polycom extension, 14Khz audio bandwidth
G.722.2 - 6.6Kbps to 23.85Kbps. Also known as AMR-WB. CELP 7Khz audio bandwidth
G.723.1 - 5.3/6.3 Kbps, 30ms frame size
G.726 - 16/24/32/40 Kbps
G.728 - 16 Kbps
G.729 - 8 Kbps, 10ms frame size
If you're using G.711 or G.722 then you may see why you're having broken outgoing audio.
Ask your SIP provider if they can set your outbound to G.726, or one that uses less bandwidth.