BT will only guarantee to provide ring current for a maximum REN of 4 - although in practice it tends to be a little higher given that most lines are not 10 km long and the loop resistance does not affect the line enough to bring the limit into play.
Any device with an electronic ringer and power supply, such as a DECT cordless will have a REN of way less than 1 and whereas it was common practice to label devices with a REN of 0.5 it seems to have fallen out of use and they default to 1. Even a standard electronic ringer is less than 1. So, everything should be fine.
If a problem develops you might hear a very short ring at the start and then some "tinkling" from te electronic ringers. If it does happen, try unplugging one or two 'phones and calling the number to see what happens.
M H C
taurus excreta cerebrum vincit