I want to hide providers IP

The only way your buyer can see the providers IP address is if you are allowing reinvite therefore sending the RTP packets directly between the buyer and seller. If you select on the provider or the buyers device Media control - can reinvite/transfer = no your server will stay in the media path and they will not see eachothers IP address.

Bare in mind that by doing this you are using more bandwidth for the call and this may reduce call quality depending on the location of the 3 legs involved.