We are using the Spectralink phones as well, I found out that IGMP snooping was preventing the multicast PTT. By default this is turned off on the 1230's but on the 1240's you enter the global command "no ip igmp snooping vlan "
I was running into this same issue with 7921/25s and Push to Talk app I wrote for a customer. Worked great for them over the past few years across 16 stores but started having problems as they were adding new stores. Thought it was that they switched to 7925s but turned out to be the newer access points(1240s) all the other sites had 1230s.