Let's start with the timers. You have the 'answer too soon' timer set to 8 seconds. Is this user experiencing the mobile phone ringing once and when they pick up the call drops? If so, the 8 second timer is too high (the default is 1.5 seconds). You can test this by changing the voicemail policy to "User Control". With User Control, the user picking up the SNR call will be asked if they want to take it or not.
If the phone is ringing once and then stopping regardless of whether the user is picking up or not, the "answer too late" timer of 19 seconds may need to be adjusted higher.
If neither of those solutions works, I suspect some kind of signaling issue through the gateway and we would need to see debugs. Try the other two things first.
Maren