I have changed the dial plan and it is still performing in the same manner. Here is the dial plan I used.

(800|*x.|**|x*x.|xx*x.|x.*x.*x.*x|[3469]11|0|00|[2-9]xxxxxx|1xxx[2-9]xxxxxxS0|xxxxxxxxxxxx.)

With either the new or old dialplan, when you dial 800, it says connected and appears like it is paging. I have included the xml from the phone.

Well, it seems the phone is paging. And it works for me on SPA50x. So you either ...

1. hit SPA51x specific firmware bug

or

2. phone works OK, but switch is not passing multicasts to non-subscribed ports

I have no SPA51x thus I can't test & confirm or reject [1]. As I claimed already, syslog may reveal more.

According [2], some switches recognize two classes of multicasts - those passed with no subscription to clients and those passed to subscribed ports only. And some switches use even more complicated algorithms. Consult your switch documentation to make sure the particular multicasts are passed. Also, ports LED should not be considered reliable sign of traffic. Use ingress port statistic to verify the packets are passed. Or better, just connect phone to a computer with no switch (using crosswired cable) to verify the multicasts are send.

Ok. I have tried several more options. First I am using Singlewire software to create multicast traffic between two computers on the same switch and tried the same ports as well. It works great. I put Wireshark on a notebook on the same switch and I can see all the traffic destined for 224.168.168.168. When I use the computers, I get traffic, when I dial 800 on the phones, no traffic. I changed the dial plan on the phone to (x.), which I believe means everything, correct? So I don't think a dial plan is stopping me.

I have put a SPA525G with 7.6.1 on the same environment with the same multicast script and no traffic. I also have a SPA509 with 7.5.6a and still no traffic.

Finally I have reached into my crap pile and pulled out a Ethernet Hub to completely rule out the switch; after finding some power adapters to power up the phones I tried it. It should bridge all traffic; again multicast from computers and none from phones. I do get multicast traffic from the phones that I see with Wireshark for dhcp requests and tftp configuration requests, just none when you attempt to page. Basically dead air!

This appears to be very simple and should just work, so it must be another configuration issue. Are there any other configuration that I may have missed? Do you have to setup the multicast address under the Linksys section?

Are there known issues with 7.6.1 for multicast on the SPA51x or SPA52x or 7.5.6a on SPA50x? 

"Ethernet Hub" is good test. Now we can considered confirmed the phone is sending no multicasts.

But all I can tell you is - it works for me on SPA508/7.5.5. As far as I know, there's no change with neither 7.5.6 nor 7.6.1

Well, we have complex configuration, I changed so many options, but if I remember correctly, the multicast paging require configuration in Linksys section only.

Once I will return to my test lab I will do some tests.

Note I called two times for syslog&debug as it may (but may not, of course) reveal the cause. It still apply.

Are there known issues with 7.6.1 for multicast on the SPA51x or SPA52x or 7.5.6a on SPA50x? 

No, but I suspect that almost no one is using such feature.