DTMF Tones on Aastra 67xxi phones

We recently installed a number of Aastra 6737i and 6735i phones. We are running FreePBX 2.11 and the Aastra XML scripts. (not sure what version.) The phones have firmware version 3.2.2.xxxx. The problem we are having is that when we press a number key, the DTMF tones being generated by the phones are very short. In many cases, when entering a password for voicemail, the tones are so short that the VM app cannot make out which number key was pressed. (I have another system with a 6757i on firmware version that does not have this problem.) Has anyone else experienced this problem?