Inconsistent connections to cell phone carriers

We've noticed a lot of inconsistency when calling cell phones from GoTo Connect (using any and all of the following... hardware phones, soft-phones, mobile app)  Sometimes, the calls are fine; they ring and go through normally.  Other times, there is a 15-30 second delay before you hear any ringing or indication that the call might be going through.  Sometimes you might hear a delay, and then the call drops.  So far we haven't been able to definitively tie this behavior to any particular cell carrier, but it does not appear to be isolated to a single carrier as best we can tell. 

When the cell phone recipient also has a GoTo Mobile app, we've noticed calls are perfect every time even when they are very inconsistent when calling the same person's cell phone number directly.  This seems to indicate that the problem is between GoTo and the cell provider, but we have no way to prove that.  

Is anyone else having a similar experience with calls to cell phones from GoTo Connect?