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Finally, in almost all cases, these are additional services that have no actual connection to your regular phone service. Some can receive calls from landline phones or send messages via SMS, but they generally don't actually do so using your regular mobile number, or you would need to adopt their number as your mobile number.
There's Got To Be Another Way!
I'm an Open Source guy beyond just software. If there is a standard way of implementing a technology, whether or not there is a .deb package for it, that's probably going to be the way I'll go. Luckily, in telephony, there is a standard called VoIP.
However, this isn't your granny's Magic Jack. I don't think that much of this is possible through a prepackaged residentials VoIP solution because they basically provide an opaque service which uses the VoIP protocol on the backend, but the front-end is generally only exposes a subset of what would be possible with that connection. There are, however, many VoIP providers around the internet that will sell you a line and service, but how you access that line is up to you. There are, a sub-set of those providers that will provide SMS services as well and often at no, or low, cost. So, this is not a free solution, but it comes in a lot cheaper than paying for a regular cellphone plan. Because VoIP is completely done over internet traffic any connection will do. Of course in order to use a phone on-the-go a mobile data plan is still necessary, but you can get a few gigabits on a data-only (tablet) plan from certain Canadian carrier. I have 4GB for $20 on a promo plan from Cityfone; a very resonable price by Canadian standards. That, plus the regular service, a couple of extra lines, taxes and small usage fees and I'm in for about $25 per month.
Any Number On Any Device
First I'll explain the extra lines. One of the cool things about VoIP is that it supports registering many numbers very easily. My primary line is a flat-rate (unlimited minutes) number based here in Ottawa that I actually use for all of my calls. All of my outbound calls originate from this line and all of the other numbers forward to this one.
The first extra number is one from back home in the same exchange that most of my relatives are used to seeing. This has no immediate value to me, it costs a bit extra and, but it does allow them to call me without them needing to pay long-distance charges. I call them using that number to limit confusion.
The other number is the toll free number that you will see on the home page of this site. This allows me to keep calls from known callers and unknown callers seperated and prevents easy scraping of my personal number from the internet. It's been my experience that telemarketers are less likely to call other toll-free numbers; that said, I do direct calls for this number directly to voicemail. There are some exceptions to that, of course. I can write rules to redirect specific callers who use that number to my primary number which will then ring.
Finally, while not a part of my own VoIP service, I also have my work number forwarded to all of my devices as well. It goes straight to voicemail outside of working hours, but it does allow me to work from home or from the road, if necessary. All of my other numbers actually turn to voicemail overnight as well, so I don't have to worry about muting my phone or completely ignoring calls.
All Devices For Any Number
So, all of the numbers can be answered from one VoIP device, but what device is that? Why choose, I say. There is, of course, several SIP clients for Android that can be used to receive VoIP calls. The nice thing about using everything over data is that my phone doesn't stop working when I lose signal, so long as I still have access to Wi-Fi. This also means that I don't actually need to have a SIM card in my phone at all. This is useful because I actually have a second, identical Nexus 5. The battery is terrible, so I can always have one charging. They sync automatically, so it doesn't really matter which one I pick up except that I will need the one with the SIM card if I'm going to stray from Wi-Fi.
But I was just mentioning that I don't want to be locked to any one interface, so the same interface on a second device isn't really an improvement. Again, because it only relies on the internet, not a specific cellular network, I can just as easily connect on basically anything with a network connection. SIP applications come in all forms, including for smartphones, all desktop operating systems, tablets and embedded systems. My primary calling device is actually my ThinkPad running Debian with Linphone. I have the same setup on my desktops at work and home.
Despite there being a GUI application for Linphone, I like commandline tools, so I actually use that most of the time. This means that I could just as easily place and receive calls without loading a desktop. While I haven't taken this to an extreme yet, I have thought about setting up a Raspberry Pi inside an old fashioned phone to use at home as well, or perhaps setting up a couple of omni-directional mics around my appartment so that I could have ubiquitous speakerphone. If I end up doing this, I'll certainly document it, but for now I enjoy standing at my desk with my extendable mic arm and studio headphones for most calls.