PC to PC or CELL TO PC, etc
You can just use a program like Skype to send text, speech or video. If you want to access regular landline telephones, you would pay to dial out. The cellphone side could use their data plan or cellphone plan to pass the data.
FREE CALLS – NO EXTRA HARDWARE
You might look into Google.Com/Voice. Get a free number even if it is not in your city or area code. You can then use your PC to have GV call you and another number, connecting them together at no charge to you. You can call your GV number and have Google call another number on your behalf.
FREE CALLS – WITH HARDWARE – SOME COST
Buy a MagicJack Plus for perhaps $50. The regular MagicJack required your computer to be on, the Plus does not. You get the first year of calling free. Then you pay $30 a year or $20 a year if you pay for 5 years in advance.
Buy an OOMA for perhaps $150 and pay just taxes (a few dollars) every month of perhaps a couple of dollars. Some options for the Ooma require an upgrade to service and a monthly fee.
PAY PER CALL
Sign up for a VOIP account with a provider like Voip.MS or CallCentric, etc. You will have a choice of free inbound, pay per minute inbound, a bundle of minutes inbound/outbound, and so on. There are many options and you should look for setup fees, monthly charges, cancellation charges, whether they supply hardware or you bring your own hardware. You will also want to see what charges there might be to have them buy you a phone number or to host one or port one in that you already own.
The account with the VOIP provider may be free to have or may require an upfront funding. Cost for calls may be in the penny or two a minute rate.
You should review your calling habits and determine if you need to call internationally and/or how often you receive calls or make calls. This will help you compare plans. As you look you may start to see that some plans are not real bargains and others may look good until you find monthly charges.