We have some GRP2602G phones, and I have found that the template in 3cx does not set the first two line keys. You can set them via the phone admin UI (until a refresh of the 3cx settings occurs), but what is shows as Key1 and 2 on the 3cx phone configuration>BLF page actually starts as number 3...
We have two brand new Grandstream GRP2613 phones and I ran into a roadblock as it appears as though I can't provision it with either v15.5 SP6 or v16 (latest release).
I've checked 3CX and they even have a "guide" on how to provision GRP61XX phones, but it looks like that phone template isn't...