IraDialer uses 3rd party SIP trunk service providers to send calls to PSTN. In order for this to work, the network where IraDialer is deployed should add the necessary firewall rules to allow SIP and RTP traffic.
Below are the links from some popular SIP trunking service providers which are usually used with IraDialer. Refer it and ensure that the same is allowed in the firewall rules/security group settings.