Troubleshoot Clock Synchronization (Background ticking)

[Iniesta]

Hello,

I have background ticking on GSM end when they/we trying to reach each other. On PBX end all clear. But when I call from cell to Auto Attendant or any other DN, I get backround ticking.

If its clocking issue, how can I start troubleshoot it, any useful advices would
Be highly appreciated.

System: Meridian 61C
Have both PREF and SREF.
PTT is Master, our PBX is slave.

Any other information missed can be provided.