ZipZap is aimed at people who take a lot of calls, sometimes concurrently: One feature I wasn't able to test in ZipZap was the ability to manage 10 calls at the same time — who does that?! Perhaps the best part is the ability to walk away from your iPad (or turn it off) and not have to worry about your smartphone ringing due to a business call as you would with other services. But you can always set up a forwarding number if you're worried about missing calls.
For $25 a month (paid yearly) you get one phone number with unlimited voice minutes and text messaging. If you prefer to pay on a monthly basis, the cost jumps to $30 a month. The free 30-day trial doesn't require a credit card.
Line2's key feature is that it's cross-platform: the company offers an app for iOS and Mac, in addition to Android and Windows. Line2's Personal service, for $10 a month, is geared towards someone who doesn't need an unlimited voice plan or extra features like auto-attendant or a toll-free number. In addition to its allotment of 5,000 outbound minute allotment, you also get unlimited text messaging and inbound calls, and access via any Line2 app.
For $15 a month, the Business plan gives you those same features along with auto-attendant, after-hours call handling, call forwarding, and a few other extras. (Both plans have a free 7-day trial.) The iPhone app you use with either plan is great — since it doesn't connect to your phone's Messages and Phone apps, it truly keeps your work conversations separate from your personal life.
I've yet to find a single solution that's as seamless to use as the phone number already attached to my iPhone, but some of them are very close — and I appreciate how they all eliminate the need for a second device. If you're looking to experiment with a second phone number — even just temporarily, like a throwaway number for selling your couch on Craiglist — you can always try Burner, which offers pay-as-you-go credits.
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