I have noticed some locked topics regarding the calling of 0800 not being able to connect I am having the same issue but not all 0800 numbers you should be able to see from my calling records. 0800 14 xnet works fine but very long waiting time to log a fault gave up waiting.
One of the most common problems is too many digits in the number dialled like those advertised number-names "0800 helpdesk". This would translate to a dialled number of 12digits. 080043573375 Most times this means the number wont route as 0800 numbers are actually 10 digits. 0800 435 733
Try dialing 0800 123 456 as a test, then try dialling 0800 123 456 78. If the second call fails its just a dial plan issue.
I see you are using a pap2t on openvfx, my first suggestion would be to use it as a certified device instead to avoid these kind of misconfiguration issues. There are a few things wrong, first of all your ata is trying to negotiate a bunch of codecs : m=audio 35452 RTP/AVP 18 0 2 4 8 96 97 98 100 101 a=rtpmap:18 G729a/8000 a=rtpmap:0 PCMU/8000 a=rtpmap:2 G726-32/8000 a=rtpmap:4 G723/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:96 G726-40/8000 a=rtpmap:97 G726-24/8000 a=rtpmap:98 G726-16/8000 a=rtpmap:100 NSE/8000 a=fmtp:100 192-193 a=rtpmap:101 telephone-event/8000 a=fmtp:101 0-15
I would suggest keeping it simple using this order :
1. G711a 2. G711u 3. G729
Second, your RTP packetization time (p-time) is set incorrectly you are sending us :
The incorrect RTP packet size will be the ultimate case of the issue, there are at least a couple of other threads on here from the past from people who have encountered the very same issue due to incorrectly configured hardware.