A = No
B&G = Yes depending on the positioning of the phones in relation to the wireless devices. Trial and error will tell.
"Should I upgrade our phones to 5.8ghz?"
Only when interference is evident and unavoidable.
I have a D-link DI-624 wireless network. Will a panasonic 2.4ghz cordless phone interfere with my wireless network? Should I upgrade our phones to 5.8ghz?