What is the maximum voltage that a RJ9/RJ10/RJ22 can carry?
I thought you were way off with 1.5 A but a browse through Farnell's list of RJ22 connectors found this.



Figure 1. Extract from MTP-44U -  CAT5 RJ22 MODULAR PLUG, 4 POSITION, 1 PORT.

The voltage isn't a problem as they have to take telephone ringing voltage which is about 100 V AC. I was surprised at the 1.5 A current rating but be aware that the you'll have to have the same rating on the socket and your conductors will be rather small for that current too.