Are you using one FPL account for 2 ATA devices? If so, that is your problem. Your freephoneline account has 2 outgoing trunks and only 1 incoming line. So if you have 2 ATA devices running the same account, you can make 2 separate outgoing phone calls just fine, but the incoming call will be flaky, because it is split between two ATA devices. This was set up to allow for call waiting to work, not for 2 separate lines.
Assuming the above is not the issue, I strongly believe you are getting flaky results because you have a router with flaky firmware. Fortunately, there is an available OpenWRT firmware for your WRTU54G-TM. I strongly recommend you install it.
http://wiki.scottn.us/wrtu54g-tmWith this flaky WRTU54G-TM firmware, I am shocked the PAP2 works fine, and the HT-702 is causing you problems. Usually the PAP2 is the prima donna, and the HT-702 just works.
Still there as a potential for conflict. It is never good to allow the router to figure it out. You should map ports for each device.
By default the PAP2 wants RTP ports 16384 to 16482, so get the router to forward these to the PAP2. As already stated, the default RTP port for the HT-702 is 5004, so get the router to forward 5004 to the HT-702. Make sure each respective ATA is set up to accept those ports. Though it should not matter, for good measure, Map port 6060 as a SIP port for the HT-702, and make sure the HT-702 and router are set up to accept 6060 as a sip port. After you save settings, and power down of the router ATA and modem, see what happens.