I wouldn't attribute a lot of abandoned calls to a gateway or network problem first. What are your call volumes and average wait times? Abandoned calls means the caller made it into queue and the call was lost while they are waiting. My first thought for a lot of abandoned calls is a spike up in call volume or not enough agents to answer calls.
after some deep investigation i found that PRI from telephony service provider gets frequent disconnections that's why i got lots of abandoned calls also i have some delay in network the client had poor design to his network