Can someone ELI5 SIP ALG?

I recently had a site that was basically a test environment for a specific type of business. They were testing out the phone install process and actual service from different VOIP providers so they had a few different systems running on their equipment. This wasn’t a typical site, I had to mdf access, just a switch where I was to do my work. Other providers were provided with the same, a Cisco 2960-X with an uplink to their network. The topology was explained to me as Centurylink fiber with different broadband providers feeding to a Cisco router to a Cisco 2960-X switch which then fed to our test install switches. They did not share much more information about the network setup and upon connecting to the switch I had dhcp IP but no internet connection, could ping gateway but no outside traffic. They made some changes and proceeded to provide me internet access.

Now before the phone installation a readiness test is ran to make sure the network is good, specifics of this is not available to me but I know it checks bandwidth as well as possible network incompatibilities. SIP ALG came up but support was unsure how it’d affect our install, the readiness test just flagged it. The phones were obihai VOIP phones and almost everything worked properly, except for call parking and transferring. Configurations were checked and working configs were used to no success. SIP ALG could not be disabled due to another provider requiring it but it was determined that was most likely causing the issues we experienced. My question is what exactly is SIP ALG and does anyone have similar experiences? I’ve done my basic read up on it and I understand the big picture of it but I’ve never encountered it in the field and am not enough network knowledged to understand why this might’ve only caused issues with these features. There was no other equipment installed, the VoIP phones were wired directly to the switch and were PoE if that makes any difference. No device in between and afaik it went from switch to switch to router though from my experience there would usually be a firewall somewhere along the line. I know this is a rather broad question with little info but this case piqued my interests