Pepwave Surf Settings and VOIP

I had switched a client from an off the shelf consumer grade router to a Surf unit and copied the settings verbatim from one to the other.

I ran in to an issue where some calls were one way audio and for some reason they just could not call toll free numbers. The sip provider told me to check the router and I did and this went back and forth for a solid 48 hours. Finally I remembered there was a sip alg setting that I could change.

I flipped it from Standard to Compatibility mode and it started working immediately.

Why is that? What’s the difference? Why wouldnt the default be compatibility mode?

I forwarded all the ports and everything was set exactly the same but that one setting fixed it.

I’m lost.

Essentially some carriers require SIP ALG to be on, some require it to be off.

Standard = SIP ALG On
Compatibility = SIP ALG Off