Sorry for bumping my own thread but we found a solution, so just in case anybody else stumbles upon this post in the future:
We found that the new 7.x Horizon Client with Skype for Business media pack handles SIP in a different way to the previous Microsoft Lync VDI plugin. We had some weird issue that was either caused by the SBC or SIP provider, and we were able to resolve in the SBC.
SBC: Audiocodes 2600
SIP: Virgin Media Business
From within the SBC, IP Profiles
Remote Early Media - change to Not Supported (was Supported)
Remote Update Support - change to Supported Only After Connect (was Not Supported)
Both changes were needed, since doing this we haven't experienced a single dropped call. Both changes do not need a reboot of SBC and we were able to perform the changes during the day without any impact.