With voip.ms, you really just pay for the DID - either for per minute or unlimited usage. If you do the unlimited, it's around $5/month. I have a bunch of lines and don't use any of them much, so I keep them all on per minute and pay the negligible fee. If you have a number come in and then ring out to your cell, you'll be paying per minute each direction... but if you have them ring to a pbx or to several SIP phones, that doesn't cost anything. It also doesn't cost anything to use their ring groups, auto attendant, etc - aside from the per-minute while with the caller.
I've done them both ways - both with my own PBX, or with multiple SIP phones connecting directly to them. Right now I'm actually using a hybrid - I have one number that trunks into my PBX, rings my standalone SIP phone that'd directly connected to them, and rings my cell phone. For <5 phones in a home, you can very easily just directly connect them to voip.ms and use their features to create even a pretty fully-featured PBX if you really wanted to. My big reason for the on-site PBX is 1) I already had it, and 2) I can do some more advanced things, like use the built in FXS port to connect to my fax (my wife is in the medical field which is tragically behind the times). I've also done PBXs for years, using PlugPBX, Raspberry Pi PBX's, 3CX on Windows, FreePBX VM's, etc. Today I have a grandstream one that's meant to plug into a PRI or SIP Trunk and is way overkill - it'd support > 200 extensions, conference bridges, fax over IP, etc... but it's a nice 1U form factor that's powered by POE so it's very simple. I even have a spare on the shelf too that I need to sell (they go for about $800).
For phones, I really love my T48G Yealink phone... but for walk-around style phones, the grandstream DECT phones are very affordable and just like any cordless - one base will support I think 5 phones, plus they have a DECT extender if you need to extend range. If you want to spend a little more, you can get into Wifi phones as well - I just haven't taken the leap for wifi because they sometimes struggle with switching AP's and doing the handoff while on a call, and depending on where I am in my house, I'll be between at least 3 different APs. I've used SNOM M3's and they're terrible... or the Yealink cordless isn't bad. You can also use like a linksys ATA or grandstream handytone ATA to use any old analog cordless phone if you want - and/or even throw a handitone ATA into a ring group and use it to plug into your TV receiver for the old Caller ID on TV features many cable/satellite boxes have.
I'm not a big tinkerer, but I like the flexibility... and being able to have the free voicemail and all the fancy ringing patterns I like is pretty handy - and without any special fees other than that basic per-minute cost. In fact, if you had 5 SIP phones around your house hooked to Voip.MS's PBX and you made internal calls, you'd never get charged for them.