Dear Dinesh,
this issue is almost everytime caused by number range inconsistencies in the ERP system.
Please check the following:
In order to avoid this problem need change the "Current Number" of Contact Person number range in R/3. Goto R/3 Txn. SNUM. Enter Object = Partner. Click on 'Number Ranges' button. Click on 'Status' button. Here you need to enter the correct Contact Person Number. This number must be greater than last Contact Person number in CRM mapping table CRMM_BUT_CONTNO.
Best regards
Thomas