All of these ports do not need to be opened for just Mitel phones, the ports needed are clearly laid out in the manual.
The port ranges needed for Mitel phone Voice Traffic are: 69, 20001/UDP; 6800-6802/TCP; 3998 and 3999/TCP, 50098-50508/UDP; 6004-6261/UDP
(The other ports mentioned in the previous post are for various networking and Inter-Tel branded IP phones)
Then set the phone programming in the 5000 IP Settings to NAT, and in the phone boot and hold 7 to put the phone in Teleworker Mode and set a New IP address of the public IP that is routed to the phone system.
You also need to set the NAT IP address properly in two placed in the 5000, go to System->IP Settings and set the System NAT IP Address to the public IP address mapped above, and in System->Devices and Feature Codes->IP Connections->P6000 set the NAT IP Address to the same public IP.
Ports needed to administer the system remotely will vary somewhat by software revision, older system use 4000/TCP for programming and newer systems use 44000/TCP, all systems use 443/TCP for Admin Web Page access and 22/TCP for telnet access to low level functions.
Bandwidth usage will vary based on the Codec used, the default G.711 uses about 90Kbps (bi-directional) for a voice call, and G.729 uses 35Kbps... the bandwidth is rarely an issues, latency and delay are the big problems with VoIP on remote phones. Changes to this are made based on the Call Configuration assigned to the set.