We found that hiring people who already understand the space you are selling into is best. Otherwise the cost of ramping up and training is really high. It took us almost a year and 5 hires to find the right hire. For us we're selling software for SDRs, we tried to hire and train and that failed flat. Now we hire people who have been SDRs in other companies, and they understand the pain points of SDRS, and can sell to other SDRs.
We tried outsourced SDR twice and wasn't able to get them working. Outsourced SDR team works better when you already have a working SDR team in house, and you just want to scale it, or if your product is easy to sell, you have proven it and you can give them the scripts that you use.
Otherwise, outbound strategy takes time tuning. What's the right message, what's the right ICP that will respond to that message. You need to be really on top of it. If you have outsourced SDR team, it can work if you treat them like an in house team and work with them really closely.