Solid is better than stranded, but the difference is so small on a short cable as to not be worth worrying about, but the OR guy was right, the internal IDCs are designed for solid, not stranded, cable. I use solid for all cables I make up that aren't going to be flexed in use, not really because they perform very slight better, but because they are far simpler to crimp into connectors.
The cable from the NTE5 Mk3 to the HG612 is also a custom made one, with an RJ45 at the NTE5 end (RJ45 plugs fit a lot better than RJ11 into NTE5 boxes, and the box is designed for an RJ45, even though it's often used with an RJ11). The other end of the same cable has an RJ11 fitted to fit into the HG612 (which will only accept an RJ11). It's a fiddle to get an RJ11 to fit to a short length of Cat5e, but it can be done by cutting off the unused pairs and being patient when inserting the cable into the plug, prior to crimping.
For anyone that wants custom cables, there's a guy on ebay, with a YouTube channel too, that can make up any cable you want. Mind you, a decent crimp tool isn't expensive and the cable and connectors are cheap, so it's worth learning to make your own. The guys YouTube channel is
https://www.youtube.com/user/mymatevince and is worth a watch. By way of payback, I buy connectors etc from his ebay store.
