This could be due to re-invites or due to your router, ATA or all three.
Are you using a STUN server? Do you have an outbound proxy configured? Is this a softphone, ATA or a hard phone?
Have tested to/From other destinations? Has the person that has dialed you tested to other destinations? This could be a problem with the PSTN number he is using as well..
You can use STUN with no port forwarding and NAT MAPPING enabled and NAT KEEP ALIVE enabled
OR
Not use STUN and forward the appropriate ports. (if you use STUN with Port forwarding there is no guarantee of the actual ports used, so no STUN with port forwarding). The pors you need to forward are the RYP port range and the SIP port as defined in the ATA.
In either case do NOT use an outbound proxy
Does your router have SPI or other security such as a hardware firewall enabled? If this is a softphone on windows disable any software firewall and security software and test again. Some SPI and firewalls are very bad for VoIP and it seems that at times it will start denying packets after a re-invite because the media starts coming from another IP and the router may interpret this as "unsolicited data", because all previous communication when to a different IP
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