Hey there! DIGITS is still alive and well and if the phone has multi-line options, it is preferred to be used that way. The errors you are getting are going to be network side and we would actually need to open a support ticket with our engineers for this. While the folks at the stores are super knowledgeable, it is true that they may not work with DIGITS every day. The best bet would be for you to call us so that our Team of Experts (folks that see all kinds of issues every day) can do a deeper investigation on the network side and open the ticket if needed.
You are not alone, remv. I'm experiencing the same issue. I called and they had me make sure I didn't experience any trouble with the DIGITS app, just when I used multiline, which is the case. I still need to call back to confirm that it is just when the multi-line settings are on. Who knows what they'll be able to do to fix this or not. I'm sort of just resigned that I'll be stuck with the digits app.
I have got to think this isn't just a one-off.