I only just found the thread about iTalk and asterisk here after leaving slingshot for this very reason, it was the straw that broke the camels back.
When we initially signed up to iTalk 6 months ago they messed up porting our number and lost our number. This ended up with us calling them every week for 3 months until they eventually got it sorted. I mus say Snap where quite helpful in the situation as they where the only people who could re-register the number and pass it back to Slignshot given it was Slingshots mess. They did refund us for the 3 months without a phone but it hardly made up for the situation.
Then we discovered their website was blocked because of a security issue for people with non slingshot IPs (we are with Snap for internet) detailed here. They then told us it was all fixed now but when I contacted Slignshot support they conceded it wasn't.
Finally the phone stopped working all together because of the asterisk issue. At this point I was in no mood to deal with more problems so jumped ship.
We have since moved to 2talk and are very happy so far. Porting was simple and easy and we where kept well informed. Although prices are slightly higher depending on how much and where you call but in my opinion well worth it.
I must say I was sceptical of slingshot from the beginning but a friend persuade me and I didn't think they could be this incompetent. This may well be a isolated case but I cant recommend slingshot to anyone and in fairness I have only been with 2talk a week.XX