The following applies to Android 2.2 on Motorola Milestone 2.
We can dial out and receive VOIP calls from our smartphone (eg Nokia, Android, etc.). The primary benefit is saving IDD and roaming charges when we travel abroad.
The prerequisite is that we have subscribed an VOIP account from a Internet Telephony Service Provider (ITSP) and ideally have public internet access for free (eg, wifi in coffee shops). To make an IDD call (eg to our home town) at low cost, we dial over the internet to the ITSP that eventually routes our call to the destination, bypassing part of the telephone network and thus saving call charges.
The ITSP is usually a telecom sevice provider that operates a VOIP gateway to serve calls from the internet side and routes to the telephone switch network. They usually charge account maintenance fee monthly and termination fee per call basis. Local termination is usually free while overseas termination are almost charged the same as if calls were originated locally.
The term VOIP (voice over IP) describes the mechanism and protocol we send voice over the internet. It centers around SIP (session initiation protocol) which is now the industry protocol standard of transmitting voice over packet switched network. Our smartphone has to become a sip compliant device in order to make VOIP calls. It is surprisingly easy as installing an SIP client apps would suffice.
There are a number of SIP client apps on Android. I installed Sipdroid (http://sipdroid.org/ ) on my MS2 because it is free and simple to use. I take a VOIP account from ZONETEL (www.zonetel.com) (one of the HK based ITSP) as illustration.
In the first place, I configure the VOIP account in Sipdroid. The VOIP account just consists of three information: 1) the ITSP server address, 2) account username, 3) account password
To make a call, we type the destination number and enter to proceed.
This VOIP account is bundled with a HK local telephone number for receiving call. It means we can be reached via this number even when we are abroad and the Sipdroid apps is running. In other words, it helps to save roaming charge.