It appears you are using a ring group, given the reference to ring all. How many phones are in the ring group where the ring all is enabled?
When using any ring all, the UCM must send an INVITE to each and every phone that is a member of the group. These INVITES are sent in a serial fashion which means that there will be some delay between the time the the first member receives its INVITE and the last member gets theirs. Additionally, the system still has to process the other functions of other users that are not members of the ring group and it also has to process the responses associated to the many INVITES it sent to the ring group.
I am uncertain as to the order or logic used to send INVITES to a ring group, but you might want to experiment by setting up a small ring group of 4 or so using the model of phones in question and see how they respond. You might then alter the members of the ring group order to see if the order has any effect on how the UCM sends (some what like a ring in order in that the ones at the top of the member list get the INVITE first). Doing a capture while testing will show the serial nature of the INVITEs as well as the order in which sent.