Asterisk will work it out by default, but if you use bandwidth those ports might need to be open on your firewall.
Bandwidth.com likes to use rtp up to 64,0000.
This seems to be at odds with Asterisk using 10k-20k.
A client is having mysterious one way audio on a phone or two, could this be because bandwidth sent an rtp port above 20k and asterisk didn’t process it?
(bandwidth.coms support line is in perpetual hold and their droning adds have turned my brain to mush)