Mgi ports work in pairs.
Since you have a os7200 you'll have a mgi16 or oas card, so 16mgi channels.
16x2=32 ports required, which is why it goes to 30031.
Each even port is rtp
Each odd port is rtcp

Adjusting your mgi port foward around fix this issue.
The pc port on the phone has no bearing in this case even if it was being used.