before you pay someone, turn off ALL internet (except for Vicidial->VOIP Carrier) during your next instance. if that fixes it, you need to either find your "waster" or get a separate internet connection for "surfing" for the kids.
we've handled this many times for many clients. there is no way to estimate it (and anyone who does is "guessing"). We've had the above suggestion resolve the issue, and we've had clients who have had to argue with their ISP for 30 days before someone finally came out and replaced a hardware component down the block (that's literal, I'm not exaggerating, and it has happened more than once). it's not always hard or always easy and there is no way to know what it will cost until it is fixed.
you can "cap" the price, but you can't guarantee resolution within that price.
many times your best bet is a local network engineer capable of tracking dropped packets who can physically visit your location if the simple "shut off internet" does not resolve the issue. this, however, can be prohibitive for cost as many of these so-called professionals are much better at collecting money than fixing actual VOIP problems (especially if they've never dealt with SIP before!).
of course your first reflex should be to get a 2nd carrier for testing. which I see you have begun on another post.