Memory fills up fast and multiple calls

Hello everyone, I need help on these topics
1-Multiple calls I am using Asterisk (Ver. 12.8.2) in a callcenter but sometime the operators can receive 3, 5 or 6 calls at same time and the operators can not pick up any calls. 2- Fills up the RAM My asterisk run on Ubuntu server 14.04 and Only asterisk service use more than 70% of the RAM memory that’s why the RAM memory fills up everytime. i have to restart everytime for to fix this issue