Why do phones have a range of RTP ports?

[fancyHeat]

I'm curious why there's a range of local RTP ports defined on the phones. Why don't the phone just receive RTP on one port like SIP

 

[JohnS_3CX]

Phones may make multiple calls at the same, for example when they make a conference call.

In this case, they need to use different RTP ports for each stream