Inbound PSTN calls getting dropped using SPA3102

I am using SPA 3102 with PSTN and VOIP enabled. While VOIP calls are being placed fine, inbound PSTN calls are getting dropped some times. Most callers have to call multiple times. Usually after 2nd or 3rd call connection is sustaining. For the first call, phone rings, but as soon as it is picked up, it is disconnecting the line and the caller will hear a continuous beep, beep.... Please help yo fix this prob. Thanks Luri