I am looking at using the following cfg. I don't suppose, with a quick look, you could see anything "obvious" thats missing or wrong?!?
!
call-manager-fallback
ip source-address 172.X.X.X port 2000
max-ephones 24
max-dn 40
limit-dn 7910 1
limit-dn 7935 1
limit-dn 7940 1...