People should not have to guess, when you're posting for help, what product (Freephoneline, Fongo Home Phone, or Fongo Mobile) you're using. It would also help if you would post in the appropriate customer support forum:
index.phpIf your issue is with Freephoneline, you should also be providing, at bare minimum, the brand and model router you're using in addition to the brand and model ATA.
If the issue only occurs when you call a single specific phone number (and if all other phone numbers work), and if you know the person you are calling is using a VoIP service, perhaps the problem is on their end and not yours. Tell them to look for a feature called SIP ALG or SPI in their router and to disable that feature.