All this stuff is covered in
the guide to varying degrees. What you need though is basically the ability to park the number, ideally it sounds like with SMS support. If you want to ensure you keep that number, you should probably avoid free services with no support.
Your cheapest option is probably going to be VOIPo's Cloud Number service at $36/year ($3/month), which will let you do the forwarding/virtual thing. Otherwise, you're probably looking at needing to spend around $5/month for mobile service. P'tel (T-Mo) and PureTalk USA (AT&T) would be your best options for the money at your suggested usage level, but most MVNOs don't permit call forwarding (P'tel does last time I checked, but minutes are double billed) and that doesn't address the SMS issue. Alternately, there is Truphone (T-Mo w/AT&T roaming) which would probably let you get things below $1/month, but I'm hesitant to recommend this route for you for reasons pertaining to costing them more than you'd pay by exploiting their billing structure, and good companies need to make money to remain good. If you're coming off of Sprint and still have the handset, maybe consider RingPlus, which will get you down to around $2+tax a month... but you may run into activation issues currently with the BYOSD program. Ideally, if you go the phone route, be sure to use an MVNO that'll let you use the phone you already have. If you're coming off Verizon, even though there's no cheap Verizon LTE handset service, remember that you might be able to go GSM instead (if the handset supports the appropriate bands - most LTE devices are GSM 850/2G network capable, so a T-Mo MVNO would be the best choice long term so long as network coverage supports your needs), and Sprint is now the same way with many of of their own LTE and global GSM/CDMA handsets. Just be sure to get your handset carrier unlocked before you port your number out to wherever you need to go.
Also ignore the coming advice of using Republic, these people fail to understand the bigger picture math. It's a bad idea in general, but it's even more of a fool's errand and ripoff for what you're needing. This can be done without any additional hardware outlay, and the service will be way better and cheaper for the money with anyone else. There's also the issue of number porting, as Bandwidth.com both owns Republic and is the managing number provider for Google Voice. If it won't port to GV, it probably won't port to Republic either. At most though, you'll need to buy a SIM card. Don't waste more than $15 in non-service costs to make this switch happen, and shouldn't generate any additional electronic demand or waste. Anything more deserves a face-punch.
All said though, it doesn't sound like your work phone is to be permitted personal usage... if so, it's best to keep things physically separated. Even if personal usage is permitted, remember it's not
your phone, which means your personal stuff isn't going to be private.