Probably not a straight answer you're looking for, but I can only recommand you to try a x86 virtualmachine based debian stretch build using the same asterisk/freepbx version matching your current RasPBX gateway build. If this does work without a audio delay you might have a starting point to debug the difference between ARM/X86 (usb driver) behaviour? It seems a lot of work but try virtualbox with USB passthrough of the GSM dongle, and follow instructions from my Opus Howto post to get you started:...
Thanks for including support in current RasPBX build, especially Alexander Traud for his detailed and updated github page. I wouldn't have done it without it. I do have to test though if upgrading to a newer Asterisk will overwrite custom compiled opus module or just reject the old one since it has a different version fingerprint?
UPDATE:2018-09-07 RasPBX maintainer includes Opus & PLC support with Asterisk 13.23.0 upgrade on current Stretch based image! ORIGINALPOST: This howto will describe a solution to get opus translation (encode/decode) support along the already (dumb) passthrough format support in RasPBX 2018-04-04 (asterisk 13.22.0/freepbx 14.03.13). Extended features like Opus FEC(Forward error correction) / PLC(Packet loss concealment) may/will need additional patching not yet described/tested! With RasPBX its current...
Opus FEC/PLC??? Atlhough I have been able to apply the PLC and Format patches from Alexander Traud his github page into building asterisk on raspbian-lite. I'm uncertain where/which asterisk components/modules also are affected by these patches except the opus related codec/format since the source patches apply to main/translate.c functions and don't know if it can be swapped/blend into RasPBX. I'm also not familair howto to even correctly test this FEC/PLC stuff except bad wifi connections with...
This howto will describe a solution to get opus translation (encode/decode) support along the already (dumb) passthrough format support in RasPBX 2018-04-04 (asterisk 13.22.0/freepbx 14.03.13)(Raspbian 8 Jessie based RasPBX with asterisk 13.17.1 also working). Extended features like Opus FEC(Forward error correction) / PLC(Packet loss concealment) may/will need additional patching not yet described/tested, see post 4! With RasPBX its current implementation (2018-04-04) its only possible to make opus...
Could you please post a link to the FreePBX wiki describring LDAP. Have you tried the x86/amd64 version of Freepbx maybe even on Centos distro in a virtualmachine to see how/if it works there there. If these questions cannot be answered for the x86/amd64 centos version of freebpx, they will be harder to answer on the raspbx platform(not impossibe).
Could you please explain your setup a bit more, since its unclear for me if you use the Dongle for roaming data (voip calls) or let raspbx translate voip to GSM audio calls.
There shouldn't be any requirment for any steps written in the documentation on the following link, although it may be wise to follow some of them. In otherwords it should give you a working system without any console interaction from the start after imaging. http://www.raspberry-asterisk.org/documentation/ However as soon as running "raspi-upgrade" on your fresh/clean image, things can get messy with dependencies not completing in time and Freepbx frontend gets out of touch with its Asterisk backend....