Cisco RV220W voip settings speach is going in jitter

I have a cisco RV220W firewall. But, when I make calls with my sip phones I get stutter/jitter speak is not fluent. Signal is falling away every 15 seconds for about 1 second in the conversation. So, it is hard to make a call. I think it is firewall based. Who has expierince with this firewall and can advise me what needs to be changed. Thanks alot